diff --git a/CHANGES.md b/CHANGES.md
index 0818f64cda..dbb32a6926 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -35,6 +35,9 @@ Fourth Release Candidate for Cylc 8 suitable for acceptance testing.
 
 ### Enhancements
 
+[#4896](https://github.com/cylc/cylc-flow/pull/4896) - Allow the setting of
+default job runner directives for platforms.
+
 [#4887](https://github.com/cylc/cylc-flow/pull/4887) - Disallow relative paths
 in `global.cylc[install]source dirs`.
 
diff --git a/cylc/flow/cfgspec/globalcfg.py b/cylc/flow/cfgspec/globalcfg.py
index cc40e8596b..f14d6c4c16 100644
--- a/cylc/flow/cfgspec/globalcfg.py
+++ b/cylc/flow/cfgspec/globalcfg.py
@@ -1313,6 +1313,17 @@
                       be appropriate if following the pattern
                       ``hosts = main, backup, failsafe``.
                 ''')
+            with Conf('directives', desc='''
+                :Defaults for: :cylc:conf:`flow.cylc\
+                [runtime][<namespace>][directives]`
+
+                Job runner (batch scheduler) directives.
+            '''):
+                Conf('<directive>', VDR.V_STRING, desc='''
+                    Example directives for the built-in job runner handlers
+                    are shown in :ref:`AvailableMethods`.
+                ''')
+
         with Conf('localhost', meta=Platform, desc='''
             A default platform defining settings for jobs to be run on the
             same host as the workflow scheduler.
diff --git a/cylc/flow/task_job_mgr.py b/cylc/flow/task_job_mgr.py
index 007afc0979..47dde478fa 100644
--- a/cylc/flow/task_job_mgr.py
+++ b/cylc/flow/task_job_mgr.py
@@ -1299,7 +1299,9 @@ def get_job_conf(
                 itask.platform['job runner command template']
             ),
             'dependencies': itask.state.get_resolved_dependencies(),
-            'directives': rtconfig['directives'],
+            'directives': {
+                **itask.platform['directives'], **rtconfig['directives']
+            },
             'environment': rtconfig['environment'],
             'execution_time_limit': itask.summary[self.KEY_EXECUTE_TIME_LIMIT],
             'env-script': rtconfig['env-script'],
diff --git a/tests/functional/platforms/09-default-directives.t b/tests/functional/platforms/09-default-directives.t
new file mode 100755
index 0000000000..c392186a01
--- /dev/null
+++ b/tests/functional/platforms/09-default-directives.t
@@ -0,0 +1,70 @@
+#!/usr/bin/env bash
+# THIS FILE IS PART OF THE CYLC WORKFLOW ENGINE.
+# Copyright (C) NIWA & British Crown (Met Office) & Contributors.
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+#-------------------------------------------------------------------------------
+# Platforms can have default directives and these are overridden as expected.
+export REQUIRE_PLATFORM="runner:slurm"
+. "$(dirname "$0")/test_header"
+
+set_test_number 5
+
+create_test_global_config "" "
+    [platforms]
+        [[no_default, default_only, overridden, neither]]
+            hosts = localhost
+            job runner = slurm
+        [[default_only, overridden]]
+            [[[directives]]]
+                --wurble=foo
+"
+
+init_workflow "${TEST_NAME_BASE}" <<'__FLOW_CONF__'
+[scheduler]
+    [[events]]
+        stall timeout = PT0S
+[scheduling]
+    [[graph]]
+        R1 = lewis & morse & barnaby & cadfael
+
+[runtime]
+    [[lewis]]
+        platform = no_default
+        [[[directives]]]
+            --wurble=bar
+    [[morse]]
+        platform = default_only
+    [[barnaby]]
+        platform = overridden
+        [[[directives]]]
+            --wurble=qux
+    [[cadfael]]
+        platform = neither
+
+__FLOW_CONF__
+
+run_fail "${TEST_NAME_BASE}-play" \
+    cylc play "${WORKFLOW_NAME}" --no-detach
+
+LOG_DIR="${RUN_DIR}/${WORKFLOW_NAME}/log/job/1/"
+
+named_grep_ok "${TEST_NAME_BASE}-no-default" "--wurble=bar" "${LOG_DIR}/lewis/NN/job"
+named_grep_ok "${TEST_NAME_BASE}-default-only" "--wurble=foo" "${LOG_DIR}/morse/NN/job"
+named_grep_ok "${TEST_NAME_BASE}-overridden" "--wurble=qux" "${LOG_DIR}/barnaby/NN/job"
+grep_fail "--wurble" "${LOG_DIR}/cadfael/NN/job"
+
+purge
+exit 0
+