diff --git a/cylc/rose/platform_utils.py b/cylc/rose/platform_utils.py
index 74435a5b..ea30c3f4 100644
--- a/cylc/rose/platform_utils.py
+++ b/cylc/rose/platform_utils.py
@@ -19,15 +19,17 @@
 """Interfaces for Cylc Platforms for use by rose apps."""
 
 from optparse import Values
+from pathlib import Path
 import sqlite3
 import subprocess
 from time import sleep
-from typing import Any, Dict
+from typing import Any, Dict, Union
 
 from cylc.flow.config import WorkflowConfig
 from cylc.flow.id_cli import parse_id
 from cylc.flow.pathutil import (
     get_workflow_run_config_log_dir,
+    get_workflow_run_dir,
     get_workflow_run_pub_db_path,
 )
 from cylc.flow.workflow_files import WorkflowFiles
@@ -57,7 +59,12 @@ def get_platform_from_task_def(flow: str, task: str) -> Dict[str, Any]:
         workflow_id,
         WorkflowFiles.FLOW_FILE_PROCESSED,
     )
-    config = WorkflowConfig(flow, flow_file, Values())
+
+    config = WorkflowConfig(
+        flow,
+        flow_file, Values(),
+        force_compat_mode=get_compat_mode(get_workflow_run_dir(workflow_id))
+    )
     # Get entire task spec to allow Cylc 7 platform from host guessing.
     task_spec = config.pcfg.get(['runtime', task])
     # check for subshell and evaluate
@@ -76,6 +83,17 @@ def get_platform_from_task_def(flow: str, task: str) -> Dict[str, Any]:
     return platform
 
 
+def get_compat_mode(run_dir: Union[str, Path]) -> bool:
+    """Check whether this is a Cylc 7 Back compatibility mode workflow:
+
+    See https://github.com/cylc/cylc-rose/issues/319
+
+    Args:
+        run_dir: Cylc workflow run directory.
+    """
+    return (Path(run_dir) / WorkflowFiles.SUITE_RC).exists()
+
+
 def eval_subshell(platform):
     """Evaluates platforms/hosts defined as subshell"""
     match = HOST_REC_COMMAND.match(platform)
diff --git a/tests/unit/test_platform_utils.py b/tests/unit/test_platform_utils.py
index b60fe7d4..e615982e 100644
--- a/tests/unit/test_platform_utils.py
+++ b/tests/unit/test_platform_utils.py
@@ -24,6 +24,12 @@
 import sqlite3
 from uuid import uuid4
 
+from cylc.rose.platform_utils import (
+    get_compat_mode,
+    get_platform_from_task_def,
+    get_platforms_from_task_jobs,
+)
+
 from cylc.flow import __version__ as cylc_version
 from cylc.flow.cfgspec.globalcfg import SPEC
 from cylc.flow.parsec.config import ParsecConfig
@@ -31,10 +37,6 @@
 from cylc.flow.workflow_db_mgr import CylcWorkflowDAO
 import pytest
 
-from cylc.rose.platform_utils import (
-    get_platform_from_task_def,
-    get_platforms_from_task_jobs,
-)
 
 MOCK_GLBL_CFG = (
     'cylc.flow.platforms.glbl_cfg',
@@ -211,6 +213,24 @@ def test_get_platform_from_task_def_subshell(
     assert platform['name'] == expected
 
 
+@pytest.mark.parametrize(
+    'create, expect',
+    (
+        (['suite.rc', 'log/conf/flow-processed.cylc'], True),
+        (['suite.rc', 'foo/bar/any-old.file'], True),
+        (['flow.cylc', 'log/conf/flow-processed.cylc'], False),
+        (['flow.cylc', 'where/flow-processed.cylc'], False),
+    )
+)
+def test_get_compat_mode(tmp_path, create, expect):
+    """It checks whether there is a suite.rc two directories up."""
+    for file in create:
+        file = tmp_path / file
+        file.parent.mkdir(parents=True, exist_ok=True)
+        file.touch()
+    assert get_compat_mode(tmp_path) == expect
+
+
 @pytest.mark.parametrize(
     'task, cycle, expect',
     [