diff --git a/changes.d/6571.fix.md b/changes.d/6571.fix.md
new file mode 100644
index 0000000000..308013afb3
--- /dev/null
+++ b/changes.d/6571.fix.md
@@ -0,0 +1 @@
+Disabled PEP-515-style integer coercion of task parameters containing underscores (e.g. `084_132` was becoming `84132`). This fix returns older behaviour seen in Cylc 7.
diff --git a/cylc/flow/config.py b/cylc/flow/config.py
index c438cd9105..dbf9316218 100644
--- a/cylc/flow/config.py
+++ b/cylc/flow/config.py
@@ -356,7 +356,6 @@ def __init__(
 
         # parameter values and templates are normally needed together.
         self.parameters = (parameter_values, parameter_templates)
-
         LOG.debug("Expanding [runtime] namespace lists and parameters")
 
         # Set default parameter expansion templates if necessary.
diff --git a/cylc/flow/parsec/validate.py b/cylc/flow/parsec/validate.py
index 12e2163a89..f060695b4a 100644
--- a/cylc/flow/parsec/validate.py
+++ b/cylc/flow/parsec/validate.py
@@ -1007,22 +1007,30 @@ def coerce_parameter_list(cls, value, keys):
             >>> CylcConfigValidator.coerce_parameter_list('a, b, c', None)
             ['a', 'b', 'c']
 
+            >>> CylcConfigValidator.coerce_parameter_list('084_132', None)
+            ['084_132']
+
+            >>> CylcConfigValidator.coerce_parameter_list('072, a', None)
+            ['072', 'a']
         """
         items = []
         can_only_be = None   # A flag to prevent mixing str and int range
         for item in cls.strip_and_unquote_list(keys, value):
             values = cls.parse_int_range(item)
             if values is not None:
-                if can_only_be == str:
+                if can_only_be is str:
                     raise IllegalValueError(
                         'parameter', keys, value, 'mixing int range and str')
                 can_only_be = int
                 items.extend(values)
             elif cls._REC_NAME_SUFFIX.match(item):  # noqa: SIM106
                 try:
+                    if '_' in item:
+                        # Disable PEP-515 int coercion; go to except block:
+                        raise ValueError()
                     int(item)
                 except ValueError:
-                    if can_only_be == int:
+                    if can_only_be is int:
                         raise IllegalValueError(
                             'parameter',
                             keys,
@@ -1034,6 +1042,10 @@ def coerce_parameter_list(cls, value, keys):
             else:
                 raise IllegalValueError(
                     'parameter', keys, value, '%s: bad value' % item)
+
+        if can_only_be is str:
+            return items
+
         try:
             return [int(item) for item in items]
         except ValueError:
diff --git a/tests/unit/parsec/test_validate.py b/tests/unit/parsec/test_validate.py
index 3e24bcf636..7a3b25cd2c 100644
--- a/tests/unit/parsec/test_validate.py
+++ b/tests/unit/parsec/test_validate.py
@@ -699,7 +699,7 @@ def test_coerce_parameter_list():
             ('-15, -10, -5, -1..1', [-15, -10, -5, -1, 0, 1])]:
         assert validator.coerce_parameter_list(value, ['whatever']) == result
     # The bad
-    for value in ['foo/bar', 'p1, 1..10', '2..3, 4, p']:
+    for value in ['foo/bar', 'p1, 1..10', '2..3, 4, p', 'x:,']:
         with pytest.raises(IllegalValueError):
             validator.coerce_parameter_list(value, ['whatever'])