diff --git a/CHANGES.md b/CHANGES.md
index cdc413c641..ce48c8120c 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -13,6 +13,13 @@ ones in. -->
 
 ## 2.1.0 (<span actions:bind='release-date'>Released 2023-07-21</span>)
 
+### Breaking Changes
+
+[2736](https://github.com/metomi/rose/pull/2736)
+Rose now ignores `PYTHONPATH` to make it more robust to task environments
+which set this value. If you want to add to the Rose environment itself,
+e.g. to write a rose-ana test, use `ROSE_PYTHONPATH`.
+
 ### Fixes
 
 [#2699](https://github.com/metomi/rose/pull/2699) -
diff --git a/metomi/rose/rose.py b/metomi/rose/rose.py
index be4606cffb..b167c40f8e 100644
--- a/metomi/rose/rose.py
+++ b/metomi/rose/rose.py
@@ -14,11 +14,39 @@
 #
 # You should have received a copy of the GNU General Public License
 # along with Rose. If not, see <http://www.gnu.org/licenses/>.
+
+import os
+import sys
+
+
+def pythonpath_manip():
+    """Stop PYTHONPATH contaminating the Cylc Environment
+
+    * Remove PYTHONPATH items from sys.path to prevent PYTHONPATH
+      contaminating the Cylc Environment.
+    * Add items from ROSE_PYTHONPATH to sys.path.
+
+    See Also:
+        https://github.com/cylc/cylc-flow/issues/5124
+    """
+    if 'ROSE_PYTHONPATH' in os.environ:
+        for item in os.environ['ROSE_PYTHONPATH'].split(os.pathsep):
+            print(f'extracted {item} from ROSE_PYTHONPATH')
+            abspath = os.path.abspath(item)
+            sys.path.insert(0, abspath)
+    if 'PYTHONPATH' in os.environ:
+        for item in os.environ['PYTHONPATH'].split(os.pathsep):
+            abspath = os.path.abspath(item)
+            if abspath in sys.path:
+                sys.path.remove(abspath)
+
+
+pythonpath_manip()
+
+
 import argparse
 from inspect import signature
-import os
 from pathlib import Path
-import sys
 
 from pkg_resources import (
     DistributionNotFound,
diff --git a/metomi/rose/tests/test_rose.py b/metomi/rose/tests/test_rose.py
new file mode 100644
index 0000000000..81745fd568
--- /dev/null
+++ b/metomi/rose/tests/test_rose.py
@@ -0,0 +1,44 @@
+# Copyright (C) British Crown (Met Office) & Contributors.
+#
+# This file is part of Rose, a framework for meteorological suites.
+#
+# Rose 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.
+#
+# Rose 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 Rose. If not, see <http://www.gnu.org/licenses/>.
+"""Test metomi/rose/rose.py
+"""
+
+import os
+import sys
+
+from metomi.rose.rose import pythonpath_manip
+
+
+def test_pythonpath_manip(monkeypatch):
+    """pythonpath_manip removes items in PYTHONPATH from sys.path
+    and adds items from ROSE_PYTHONPATH
+    """
+    # If PYTHONPATH is set...
+    monkeypatch.setenv('PYTHONPATH', '/remove-from-sys.path')
+    monkeypatch.setattr('sys.path', ['/leave-alone', '/remove-from-sys.path'])
+    pythonpath_manip()
+    # ... we don't change PYTHONPATH
+    assert os.environ['PYTHONPATH'] == '/remove-from-sys.path'
+    # ... but we do remove PYTHONPATH items from sys.path, and don't remove
+    # items there not in PYTHONPATH
+    assert sys.path == ['/leave-alone']
+
+    # If CYLC_PYTHONPATH is set we retrieve its contents and
+    # add them to the sys.path:
+    monkeypatch.setenv('ROSE_PYTHONPATH', '/add-to-sys.path')
+    pythonpath_manip()
+    assert sys.path == ['/add-to-sys.path', '/leave-alone']
diff --git a/t/rose-ana/00-run-basic/flow.cylc b/t/rose-ana/00-run-basic/flow.cylc
index 0b35076ef9..b0d4be2692 100644
--- a/t/rose-ana/00-run-basic/flow.cylc
+++ b/t/rose-ana/00-run-basic/flow.cylc
@@ -4,7 +4,7 @@ UTC mode=True
     [[events]]
         abort on stalled = True
         mail events =
-        timeout=PT30S
+        timeout=PT5S
 
 [scheduling]
     [[dependencies]]
diff --git a/tox.ini b/tox.ini
index 8dad1c4452..83a93c182c 100644
--- a/tox.ini
+++ b/tox.ini
@@ -13,3 +13,5 @@ ignore=
     E731,
     # no longer best practice:
     W503
+    ; module level import not at top of file
+    E402,