diff --git a/config/flags.py b/config/flags.py
new file mode 100644
index 000000000..0d62a8793
--- /dev/null
+++ b/config/flags.py
@@ -0,0 +1 @@
+ACTUALISATION = "actualisation"
diff --git a/conftest.py b/conftest.py
new file mode 100644
index 000000000..a275cd59f
--- /dev/null
+++ b/conftest.py
@@ -0,0 +1,17 @@
+import pytest
+from django.contrib.auth import get_user_model
+
+
+@pytest.fixture
+def test_user(db):
+    test_user_email = "test@test.com"
+    test_password = "test_password"
+
+    test_user, _ = get_user_model().objects.get_or_create(
+        username="test_user",
+        email=test_user_email,
+    )
+    test_user.set_password(test_password)
+    test_user.save()
+
+    return test_user
diff --git a/core/constants.py b/core/constants.py
index b580d3824..9cd05d9b6 100644
--- a/core/constants.py
+++ b/core/constants.py
@@ -1,4 +1,4 @@
-from .types import Months
+from .types import FinancialPeriods, Months
 
 
 MONTHS: Months = (
@@ -15,3 +15,4 @@
     "feb",
     "mar",
 )
+PERIODS: FinancialPeriods = (1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12)
diff --git a/core/types.py b/core/types.py
index 3e7427ef2..a32670289 100644
--- a/core/types.py
+++ b/core/types.py
@@ -1,6 +1,21 @@
 from typing import Literal, TypedDict
 
 
+FinancialPeriods = tuple[
+    Literal[1],
+    Literal[2],
+    Literal[3],
+    Literal[4],
+    Literal[5],
+    Literal[6],
+    Literal[7],
+    Literal[8],
+    Literal[9],
+    Literal[10],
+    Literal[11],
+    Literal[12],
+]
+
 Month = Literal[
     "apr",
     "may",
diff --git a/forecast/import_actuals.py b/forecast/import_actuals.py
index e769cb111..78bfe13fe 100644
--- a/forecast/import_actuals.py
+++ b/forecast/import_actuals.py
@@ -1,8 +1,12 @@
 import datetime
 import logging
 
+import waffle
 from django.db import connection
+from django.db.models import F
 
+from config import flags
+from core.constants import PERIODS
 from core.import_csv import get_fk, get_fk_from_field
 from core.models import FinancialYear
 from forecast.models import (
@@ -292,8 +296,16 @@ def upload_trial_balance_report(file_upload, month_number, financial_year):
     if check_financial_code.error_found:
         final_status = FileUpload.PROCESSEDWITHERROR
     else:
+        uploaded_actuals = ActualUploadMonthlyFigure.objects.filter(
+            financial_year=financial_year, financial_period=period_obj
+        )
+
         # Now copy the newly uploaded actuals to the correct table
         if year_obj.current:
+            if waffle.switch_is_active(flags.ACTUALISATION):
+                for uploaded_actual in uploaded_actuals:
+                    actualisation(period=period_obj, actual=uploaded_actual)
+
             copy_current_year_actuals_to_monthly_figure(period_obj, financial_year)
             FinancialPeriod.objects.filter(
                 financial_period_code__lte=period_obj.financial_period_code
@@ -307,11 +319,42 @@ def upload_trial_balance_report(file_upload, month_number, financial_year):
         if check_financial_code.warning_found:
             final_status = FileUpload.PROCESSEDWITHWARNING
 
-        ActualUploadMonthlyFigure.objects.filter(
-            financial_year=financial_year, financial_period=period_obj
-        ).delete()
+        uploaded_actuals.delete()
 
     set_file_upload_feedback(
         file_upload, f"Processed {rows_to_process} rows.", final_status
     )
     return True
+
+
+def actualisation(period: FinancialPeriod, actual: ActualUploadMonthlyFigure) -> None:
+    # get the current forecast that is being turned into an actual
+    forecast = ForecastMonthlyFigure.objects.filter(
+        financial_code_id=actual.financial_code_id,
+        financial_year_id=actual.financial_year_id,
+        financial_period_id=actual.financial_period_id,
+        archived_status__isnull=True,
+    ).first()
+
+    # work out how many period we have left in the financial year
+    periods_left = len(PERIODS) - period.financial_period_code
+    # handle a missing forecast object and assume a forecast amount of 0
+    forecast_amount = forecast.amount if forecast else 0
+    # work out the difference the actual will leave us with
+    difference = forecast_amount - actual.amount
+
+    if periods_left:
+        # floor divide the difference by how many periods are left in the financial year
+        # TODO: How should monetary values be treated with regards to rounding?
+        difference //= periods_left
+
+    # adjust the remaining forecast periods by the difference
+    for i in range(periods_left):
+        ForecastMonthlyFigure.objects.update_or_create(
+            financial_code_id=actual.financial_code_id,
+            financial_year_id=actual.financial_year_id,
+            financial_period_id=period.pk + i + 1,
+            archived_status=None,
+            defaults={"amount": F("amount") + difference},
+            create_defaults={"amount": difference},
+        )
diff --git a/forecast/models.py b/forecast/models.py
index 0670532ca..f02711154 100644
--- a/forecast/models.py
+++ b/forecast/models.py
@@ -287,9 +287,12 @@ def get_max_period(self):
 
 
 class FinancialPeriod(BaseModel):
-    """Financial periods: correspond
-    to month, but there are 3 extra
-    periods at the end"""
+    """Financial periods: correspond to month, but there are 3 extra periods at the end.
+
+    There are 15 objects in total.
+
+    The objects are managed in migrations and therefore always available in tests.
+    """
 
     financial_period_code = models.IntegerField(primary_key=True)  # April = 1
     period_long_name = models.CharField(max_length=20)
@@ -1027,8 +1030,9 @@ class MonthlyFigureAbstract(BaseModel):
     """It contains the forecast and the actuals.
     The current month defines what is Actual and what is Forecast"""
 
-    amount = models.BigIntegerField(default=0)  # stored in pence
     id = models.AutoField(primary_key=True)
+
+    amount = models.BigIntegerField(default=0)  # stored in pence
     financial_year = models.ForeignKey(
         FinancialYear,
         on_delete=models.PROTECT,
diff --git a/forecast/services.py b/forecast/services.py
index a35514671..341a9d545 100644
--- a/forecast/services.py
+++ b/forecast/services.py
@@ -1,4 +1,4 @@
-from core.constants import MONTHS
+from core.constants import PERIODS
 from core.models import FinancialYear
 from forecast.models import FinancialCode, FinancialPeriod, ForecastMonthlyFigure
 
@@ -40,7 +40,7 @@ def update_period(self, *, period: int | FinancialPeriod, amount: int):
         figure.save()
 
     def update(self, forecast: list[int]):
-        assert len(forecast) == len(MONTHS)
+        assert len(forecast) == len(PERIODS)
 
-        for i, _ in enumerate(MONTHS):
-            self.update_period(period=i + 1, amount=forecast[i])
+        for period in PERIODS:
+            self.update_period(period=period, amount=forecast[period - 1])
diff --git a/forecast/test/test_assets/actualisation_test.xlsx b/forecast/test/test_assets/actualisation_test.xlsx
new file mode 100644
index 000000000..e20c88da5
Binary files /dev/null and b/forecast/test/test_assets/actualisation_test.xlsx differ
diff --git a/forecast/test/test_import_actuals.py b/forecast/test/test_import_actuals.py
index ed0b91bce..5844e9781 100644
--- a/forecast/test/test_import_actuals.py
+++ b/forecast/test/test_import_actuals.py
@@ -3,6 +3,8 @@
 from unittest.mock import MagicMock, patch
 from zipfile import BadZipFile
 
+import pytest
+import waffle.testutils
 from django.contrib.auth.models import Group, Permission
 from django.core.files import File
 from django.db.models import Sum
@@ -11,7 +13,9 @@
 
 from chartofaccountDIT.models import NaturalCode, ProgrammeCode
 from chartofaccountDIT.test.factories import NaturalCodeFactory, ProgrammeCodeFactory
+from config import flags
 from core.models import FinancialYear
+from core.test.factories import FinancialYearFactory
 from core.test.test_base import TEST_COST_CENTRE, BaseTestCase
 from core.utils.excel_test_helpers import FakeCell, FakeWorkSheet
 from core.utils.generic_helpers import make_financial_year_current
@@ -25,6 +29,7 @@
     NAC_NOT_VALID_WITH_GENERIC_PROGRAMME,
     TITLE_CELL,
     UploadFileFormatError,
+    actualisation,
     check_trial_balance_format,
     copy_current_year_actuals_to_monthly_figure,
     save_trial_balance_row,
@@ -36,6 +41,7 @@
     FinancialPeriod,
     ForecastMonthlyFigure,
 )
+from forecast.test.factories import FinancialCodeFactory
 from forecast.utils.import_helpers import VALID_ECONOMIC_CODE_LIST, CheckFinancialCode
 from upload_file.models import FileUpload
 
@@ -306,6 +312,7 @@ def test_save_row_invalid_nac(self):
             True,
         )
 
+    @waffle.testutils.override_switch(flags.ACTUALISATION, active=True)
     def test_upload_trial_balance_report(self):
         # Check that BadZipFile is raised on
         # supply of incorrect file format
@@ -443,25 +450,43 @@ def test_upload_trial_balance_report(self):
         # Check that existing figures for the same period have been deleted
         self.assertEqual(
             ForecastMonthlyFigure.objects.filter(
-                financial_code__cost_centre=cost_centre_code_1
+                financial_code__cost_centre=cost_centre_code_1,
+                financial_year=self.test_year,
+                financial_period__period_calendar_code=self.test_period,
             ).count(),
             0,
         )
         # Check for existence of monthly figures
         self.assertEqual(
             ForecastMonthlyFigure.objects.filter(
-                financial_code__cost_centre=self.cost_centre_code
+                financial_code__cost_centre=self.cost_centre_code,
+                financial_year=self.test_year,
+                financial_period__period_calendar_code=self.test_period,
             ).count(),
             4,
         )
         result = ForecastMonthlyFigure.objects.filter(
-            financial_code__cost_centre=self.cost_centre_code
+            financial_code__cost_centre=self.cost_centre_code,
+            financial_year=self.test_year,
+            financial_period__period_calendar_code=self.test_period,
         ).aggregate(total=Sum("amount"))
 
         # Check that figures have correct values
         self.assertEqual(
             result["total"],
-            1000000,
+            1_000_000,
+        )
+
+        result_inc_actualisation = ForecastMonthlyFigure.objects.filter(
+            financial_code__cost_centre=self.cost_centre_code,
+        ).aggregate(total=Sum("amount"))
+
+        # Check that actualisation has been applied and that the total forecast remains
+        # the same. In this case it will be "zero" (or close to depending on rounding)
+        # because there was a total forecast of 0 before uploading the actual.
+        self.assertEqual(
+            result_inc_actualisation["total"],
+            -8,  # due to floor division
         )
 
         self.assertTrue(
@@ -793,3 +818,159 @@ def test_finance_admin_can_upload_actuals(self, mock_process_uploaded_file):
         file_path = "uploaded/actuals/{}".format(self.file_mock.name)
         if os.path.exists(file_path):
             os.remove(file_path)
+
+
+@waffle.testutils.override_switch(flags.ACTUALISATION, active=True)
+def test_actualisation_as_part_of_upload(db, test_user):
+    # This test is based off an example given to me.
+
+    # figures are in pence
+    figures = [
+        # actuals apr - aug
+        10_500_00,
+        5_000_00,
+        2_500_00,
+        3_000_00,
+        6_000_00,
+        # forecast sep - mar
+        10_000_00,
+        10_000_00,
+        10_000_00,
+        10_000_00,
+        10_000_00,
+        10_000_00,
+        10_000_00,
+    ]
+    expected_figures = [
+        # actuals apr - sep
+        10_500_00,
+        5_000_00,
+        2_500_00,
+        3_000_00,
+        6_000_00,
+        7_000_00,
+        # forecast oct - mar
+        10_500_00,
+        10_500_00,
+        10_500_00,
+        10_500_00,
+        10_500_00,
+        10_500_00,
+    ]
+
+    actual_period = FinancialPeriod.objects.get(pk=6)  # sep
+    # actual_amount = 7_000  # stored in actualisation_test.xslx
+
+    fin_code: FinancialCode = FinancialCodeFactory(
+        natural_account_code__economic_budget_code=VALID_ECONOMIC_CODE_LIST[0],
+    )
+    fin_year: FinancialYear = FinancialYearFactory()
+
+    make_financial_year_current(fin_year.pk)
+
+    for i, amount in enumerate(figures):
+        FinancialPeriod.objects.filter(pk=i + 1).update(actual_loaded=True)
+        ForecastMonthlyFigure.objects.create(
+            financial_code=fin_code,
+            financial_year=fin_year,
+            financial_period_id=i + 1,
+            amount=amount,
+        )
+
+    excel_file = FileUpload(
+        s3_document_file=os.path.join(
+            os.path.dirname(__file__),
+            "test_assets/actualisation_test.xlsx",
+        ),
+        uploading_user=test_user,
+        document_type=FileUpload.ACTUALS,
+    )
+    excel_file.save()
+    upload_trial_balance_report(
+        excel_file,
+        actual_period.period_calendar_code,
+        fin_year.pk,
+    )
+
+    new_figures = (
+        ForecastMonthlyFigure.objects.filter(
+            financial_code=fin_code,
+            financial_year=fin_year,
+            archived_status__isnull=True,
+        )
+        .order_by("financial_period")
+        .values_list("amount", flat=True)
+    )
+
+    assert list(new_figures) == expected_figures
+
+
+@pytest.mark.parametrize(
+    ["figures", "period", "actual", "expected_figures"],
+    [
+        # fmt: off
+        (
+            [10_500, 5_000, 2_500, 3_000, 6_000, 10_000, 10_000, 10_000, 10_000, 10_000, 10_000, 10_000],
+            6,  # september
+            7_000,
+            [10_500, 5_000, 2_500, 3_000, 6_000, 7_000, 10_500, 10_500, 10_500, 10_500, 10_500, 10_500],
+        ),
+        (
+            [10_500, 5_000, 2_500, 3_000, 6_000, 10_000, 10_000, 10_000, 10_000, 10_000, 10_000, 10_000],
+            6,  # september
+            7050,
+            [10_500, 5_000, 2_500, 3_000, 6_000, 7_000, 10_491.66, 10_491.66, 10_491.66, 10_491.66, 10_491.66, 10_491.66],
+        ),
+        (
+            [0, 0, 0, 50_000, 0, 0, 0, 0, 0, 50_000, 0, 0],
+            6,  # september
+            12_000,
+            [0, 0, 0, 50_000, 0, 12_000, -2000, -2000, -2000, 48_000, -2000, -2000],
+        ),
+        (
+            [10_500, 5_000, 2_500, 3_000, 6_000, 10_000, 10_000, 10_000, 10_000, 10_000, 10_000, 10_000],
+            12,  # march
+            7_000,
+            [10_500, 5_000, 2_500, 3_000, 6_000, 10_000, 10_000, 10_000, 10_000, 10_000, 10_000, 7_000],
+        ),
+        # fmt: on\
+    ],
+)
+def test_actualisation(
+    db, figures: list[float], period: int, actual: float, expected_figures: list[float]
+):
+    period_obj = FinancialPeriod.objects.get(pk=period)
+
+    fin_code: FinancialCode = FinancialCodeFactory(
+        natural_account_code__economic_budget_code=VALID_ECONOMIC_CODE_LIST[0],
+    )
+    fin_year: FinancialYear = FinancialYearFactory()
+
+    for i, amount in enumerate(figures):
+        ForecastMonthlyFigure.objects.create(
+            financial_code=fin_code,
+            financial_year=fin_year,
+            financial_period_id=i + 1,
+            amount=amount * 100,
+        )
+
+    actual_obj = ActualUploadMonthlyFigure.objects.create(
+        financial_code=fin_code,
+        financial_year=fin_year,
+        financial_period=period_obj,
+        amount=actual * 100,
+    )
+
+    actualisation(period=period_obj, actual=actual_obj)
+
+    new_figures = (
+        ForecastMonthlyFigure.objects.filter(
+            financial_code=fin_code,
+            financial_year=fin_year,
+            archived_status__isnull=True,
+        )
+        .order_by("financial_period")
+        .values_list("amount", flat=True)
+    )
+
+    assert list(new_figures[period:]) == [x * 100 for x in expected_figures][period:]
diff --git a/makefile b/makefile
index c5b79d072..29e353dc4 100644
--- a/makefile
+++ b/makefile
@@ -88,6 +88,8 @@ superuser: # Create superuser
 
 feature-flags: # Manage feature flags for local development
 	echo 'Add feature flags here'
+	$(web) $(manage) waffle_switch actualisation on --create
+
 # Formatting
 black-check: # Run black-check
 	$(run-host) black --check .