diff --git a/cylc/flow/prerequisite.py b/cylc/flow/prerequisite.py
index ad608af78a9..486c7e84ab3 100644
--- a/cylc/flow/prerequisite.py
+++ b/cylc/flow/prerequisite.py
@@ -16,7 +16,6 @@
 
 """Functionality for expressing and evaluating logical triggers."""
 
-import math
 import re
 from typing import (
     TYPE_CHECKING,
@@ -286,10 +285,10 @@ def api_dump(self) -> Optional[PbPrerequisite]:
                 for s_msg in self._satisfied
             )
         conds = []
-        num_length = math.ceil(len(self._satisfied) / 10)
+        num_length = len(str(len(self._satisfied)))
         for ind, message_tuple in enumerate(sorted(self._satisfied)):
             t_id = message_tuple.get_id()
-            char = 'c%.{0}d'.format(num_length) % ind
+            char = str(ind).zfill(num_length)
             c_msg = self.MESSAGE_TEMPLATE % message_tuple
             c_val = self._satisfied[message_tuple]
             conds.append(
diff --git a/tests/flakyfunctional/cylc-show/00-simple.t b/tests/flakyfunctional/cylc-show/00-simple.t
index 8e6b9156924..f96a1129268 100644
--- a/tests/flakyfunctional/cylc-show/00-simple.t
+++ b/tests/flakyfunctional/cylc-show/00-simple.t
@@ -112,10 +112,10 @@ cmp_json "${TEST_NAME}-taskinstance" "${TEST_NAME}-taskinstance" \
         "runtime": {"completion": "(started and succeeded)"},
         "prerequisites": [
             {
-                "expression": "c0",
+                "expression": "0",
                 "conditions": [
                     {
-                        "exprAlias": "c0",
+                        "exprAlias": "0",
                         "taskId": "20141106T0900Z/bar",
                         "reqState": "succeeded",
                         "message": "satisfied naturally",