diff --git a/lib/iris/fileformats/rules.py b/lib/iris/fileformats/rules.py
index f8e003f9b7..ee1c958dcc 100644
--- a/lib/iris/fileformats/rules.py
+++ b/lib/iris/fileformats/rules.py
@@ -315,43 +315,6 @@ class Reference(iris.util._OrderedHashable):
     """
 
 
-def calculate_forecast_period(time, forecast_reference_time):
-    """
-    Return the forecast period in hours derived from time and
-    forecast_reference_time scalar coordinates.
-
-    .. deprecated:: 1.10
-
-    """
-    warn_deprecated("the `iris.fileformats.rules.calculate_forecast_period "
-                    "method is deprecated.")
-
-    if time.points.size != 1:
-        raise ValueError('Expected a time coordinate with a single '
-                         'point. {!r} has {} points.'.format(time.name(),
-                                                             time.points.size))
-
-    if not time.has_bounds():
-        raise ValueError('Expected a time coordinate with bounds.')
-
-    if forecast_reference_time.points.size != 1:
-        raise ValueError('Expected a forecast_reference_time coordinate '
-                         'with a single point. {!r} has {} '
-                         'points.'.format(forecast_reference_time.name(),
-                                          forecast_reference_time.points.size))
-
-    origin = time.units.origin.replace(time.units.origin.split()[0], 'hours')
-    units = cf_units.Unit(origin, calendar=time.units.calendar)
-
-    # Determine start and eof of period in hours since a common epoch.
-    end = time.units.convert(time.bounds[0, 1], units)
-    start = forecast_reference_time.units.convert(
-        forecast_reference_time.points[0], units)
-    forecast_period = end - start
-
-    return forecast_period
-
-
 class Rule(object):
     """
     A collection of condition expressions and their associated action expressions.