diff --git a/changes.d/6433.fix.md b/changes.d/6433.fix.md
new file mode 100644
index 0000000000..92531f0d89
--- /dev/null
+++ b/changes.d/6433.fix.md
@@ -0,0 +1 @@
+Ignore requests to trigger or set active tasks with --flow=none.
diff --git a/cylc/flow/task_pool.py b/cylc/flow/task_pool.py
index 3ddf991cb9..e149630d32 100644
--- a/cylc/flow/task_pool.py
+++ b/cylc/flow/task_pool.py
@@ -1927,6 +1927,12 @@ def set_prereqs_and_outputs(
 
         # Set existing task proxies.
         for itask in itasks:
+            if flow == ['none'] and itask.flow_nums != set():
+                LOG.error(
+                    f"[{itask}] ignoring 'flow=none' set: task already has"
+                    f" {stringify_flow_nums(itask.flow_nums, full=True)}"
+                )
+                continue
             self.merge_flows(itask, flow_nums)
             if prereqs:
                 self._set_prereqs_itask(itask, prereqs, flow_nums)
@@ -2169,8 +2175,14 @@ def force_trigger_tasks(
 
         # Trigger active tasks.
         for itask in existing_tasks:
+            if flow == ['none'] and itask.flow_nums != set():
+                LOG.error(
+                    f"[{itask}] ignoring 'flow=none' trigger: task already has"
+                    f" {stringify_flow_nums(itask.flow_nums, full=True)}"
+                )
+                continue
             if itask.state(TASK_STATUS_PREPARING, *TASK_STATUSES_ACTIVE):
-                LOG.warning(f"[{itask}] ignoring trigger - already active")
+                LOG.error(f"[{itask}] ignoring trigger - already active")
                 continue
             self.merge_flows(itask, flow_nums)
             self._force_trigger(itask)
diff --git a/tests/integration/test_flow_assignment.py b/tests/integration/test_flow_assignment.py
index 5816b08527..6c0c58a875 100644
--- a/tests/integration/test_flow_assignment.py
+++ b/tests/integration/test_flow_assignment.py
@@ -17,12 +17,18 @@
 """Test for flow-assignment in triggered/set tasks."""
 
 import functools
+import logging
 import time
 from typing import Callable
 
 import pytest
 
-from cylc.flow.flow_mgr import FLOW_ALL, FLOW_NEW, FLOW_NONE
+from cylc.flow.flow_mgr import (
+    FLOW_ALL,
+    FLOW_NEW,
+    FLOW_NONE,
+    stringify_flow_nums
+)
 from cylc.flow.scheduler import Scheduler
 
 
@@ -76,7 +82,9 @@ async def test_get_flow_nums(one: Scheduler, start):
 
 
 @pytest.mark.parametrize('command', ['trigger', 'set'])
-async def test_flow_assignment(flow, scheduler, start, command: str):
+async def test_flow_assignment(
+    flow, scheduler, start, command: str, log_filter: Callable
+):
     """Test flow assignment when triggering/setting tasks.
 
     Active tasks:
@@ -102,7 +110,7 @@ async def test_flow_assignment(flow, scheduler, start, command: str):
     }
     id_ = flow(conf)
     schd: Scheduler = scheduler(id_, run_mode='simulation', paused_start=True)
-    async with start(schd):
+    async with start(schd) as log:
         if command == 'set':
             do_command: Callable = functools.partial(
                 schd.pool.set_prereqs_and_outputs, outputs=['x'], prereqs=[]
@@ -128,6 +136,14 @@ async def test_flow_assignment(flow, scheduler, start, command: str):
         # (no-flow is ignored for active tasks)
         do_command([active_a.identity], flow=[FLOW_NONE])
         assert active_a.flow_nums == {1, 2}
+        assert log_filter(
+            log,
+            contains=(
+                f'[{active_a}] ignoring \'flow=none\' {command}: '
+                f'task already has {stringify_flow_nums(active_a.flow_nums)}'
+            ),
+            level=logging.ERROR
+        )
 
         do_command([active_a.identity], flow=[FLOW_NEW])
         assert active_a.flow_nums == {1, 2, 3}