diff --git a/CHANGELOG.md b/CHANGELOG.md
index 53589708..ec98c77a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -22,6 +22,10 @@ Modifications by (in alphabetical order):
 * P. Vitt, University of Siegen, Germany
 * A. Voysey, UK Met Office
 
+18/08/2026 PR #526 for #484. Add the 'format-missing-comma' extension: accept a
+           missing comma between a character-string edit descriptor and a
+           neighbouring format item, as many compilers do.
+
 16/07/2026 PR #520 for #519. Fix truncation of a character length expression that
            contains a comma (e.g. the arguments to an intrinsic such as
            MAX) when the kind appears before the length in a char-selector.
diff --git a/doc/source/fparser2.rst b/doc/source/fparser2.rst
index c9e8f51b..eb9b556c 100644
--- a/doc/source/fparser2.rst
+++ b/doc/source/fparser2.rst
@@ -319,6 +319,23 @@ omitted, the value is implicitly assumed to be one. For example::
 For more information see
 https://gcc.gnu.org/onlinedocs/gfortran/X-format-descriptor-without-count-field.html
 
+Missing Comma in Format Specifications
+++++++++++++++++++++++++++++++++++++++
+
+Standard Fortran only permits the comma separating format items to be
+omitted in a small number of situations (constraint C1002). However,
+many compilers (e.g. gfortran, ifort, ifx) additionally accept a
+missing comma between a character-string edit descriptor and a
+neighbouring format item. For example::
+
+  100 format('a' 1x,'b')
+  200 format(15x'a')
+  300 format('a' 'b')
+
+The 'format-missing-comma' extension adds support in fparser for this
+relaxation. Note that when such a format specification is re-generated
+from the parse tree the omitted commas are re-introduced.
+
 Hollerith Constant
 ++++++++++++++++++
 
diff --git a/src/fparser/two/Fortran2003.py b/src/fparser/two/Fortran2003.py
index ecc89765..ced72700 100644
--- a/src/fparser/two/Fortran2003.py
+++ b/src/fparser/two/Fortran2003.py
@@ -10039,6 +10039,35 @@ def skip_digits(string):
     return found, index
 
 
+def split_leading_char_literal(string):
+    """Splits a string that starts with a character literal into the
+    literal and the remainder, honouring doubled (escaped) quotes
+    inside the literal.
+
+    :param str string: the string to split.
+
+    :returns: a 2-tuple containing the character literal and the \
+        remainder of the string, or None if the string does not start \
+        with a complete character literal.
+    :rtype: Optional[Tuple[str, str]]
+
+    """
+    if not string or string[0] not in "'\"":
+        return None
+    quote = string[0]
+    index = 1
+    while index < len(string):
+        if string[index] == quote:
+            if index + 1 < len(string) and string[index + 1] == quote:
+                # A doubled quote is an escaped quote, still inside the
+                # literal.
+                index += 2
+                continue
+            return string[: index + 1], string[index + 1 :]
+        index += 1
+    return None
+
+
 class Format_Item_C1002(Base):  # pylint: disable=invalid-name
     """
     Fortran 2003 constraint C1002::
@@ -10063,6 +10092,12 @@ class Format_Item_C1002(Base):  # pylint: disable=invalid-name
 
     (4) Before or after a colon edit descriptor.
 
+    If 'format-missing-comma' is specified in the EXTENSIONS list then
+    the comma may additionally be omitted between a character-string
+    edit descriptor and any neighbouring format item, e.g.
+    FORMAT('a' 1x,'b'), FORMAT(15x'a') or FORMAT('a' 'b'), as accepted
+    by many compilers (e.g. gfortran, ifort, ifx).
+
     """
 
     subclass_names = []
@@ -10070,6 +10105,36 @@ class Format_Item_C1002(Base):  # pylint: disable=invalid-name
 
     @staticmethod
     def match(string):
+        """Implements the matching for the C1002 Format Item constraint,
+        optionally relaxed by the 'format-missing-comma' extension.
+
+        :param str string: The string to check for conformance with a \
+                           C1002 format item constraint.
+        :return: `None` if there is no match, otherwise a tuple of \
+        size 2 containing a mixture of Control_Edit_Descriptor and \
+        Format_Item classes depending on what has been matched.
+
+        :rtype: `NoneType` or ( \
+        :py:class:`fparser.two.Control_Edit_Desc`, \
+        :py:class:`fparser.two.Format_Item` ) or \
+        (:py:class:`fparser.two.Format_Item`, \
+        :py:class:`fparser.two.Control_Edit_Desc`) or \
+        (:py:class:`fparser.two.Format_Item`, \
+        :py:class:`fparser.two.Format_Item`)
+
+        """
+        try:
+            result = Format_Item_C1002._standard_match(string)
+        except NoMatchError:
+            result = None
+        if result:
+            return result
+        if "format-missing-comma" not in EXTENSIONS():
+            return None
+        return Format_Item_C1002._extension_match(string)
+
+    @staticmethod
+    def _standard_match(string):
         """Implements the matching for the C1002 Format Item constraint. The
         constraints specify certain combinations of format items that
         do not need a comma to separate them. Rather than sorting this
@@ -10171,6 +10236,52 @@ def match(string):
                     Format_Item(option + repmap(right.lstrip())),
                 )
 
+        return None
+
+    @staticmethod
+    def _extension_match(string):
+        """Implements the matching for the 'format-missing-comma'
+        extension. Various compilers (e.g. gfortran, ifort, ifx) accept
+        a missing comma between a character-string edit descriptor and
+        the neighbouring format item, e.g. FORMAT('a' 1x,'b'),
+        FORMAT(15x'a') or FORMAT('a' 'b'). The item is split at the
+        boundary of the first character literal and both sides are
+        matched separately.
+
+        :param str string: The string to check for conformance with the \
+                           'format-missing-comma' extension.
+        :return: `None` if there is no match, otherwise a tuple of \
+        size 2 containing two Format_Item classes.
+        :rtype: `NoneType` or \
+        (:py:class:`fparser.two.Format_Item`, \
+        :py:class:`fparser.two.Format_Item`)
+
+        """
+        if not string:
+            return None
+        strip_string = string.strip()
+        split = split_leading_char_literal(strip_string)
+        if split:
+            # The item starts with a character literal, e.g. "'a' 1x".
+            literal, rest = split
+            rest = rest.lstrip()
+            if not rest or rest.startswith(","):
+                # Nothing follows the literal, or standard syntax.
+                return None
+            return (Format_Item(literal), Format_Item(rest))
+        indices = [
+            strip_string.find(quote) for quote in "'\"" if strip_string.find(quote) > 0
+        ]
+        if indices:
+            # The item contains a character literal preceded by another
+            # format item, e.g. "15x'a'". Split at the earliest quote.
+            index = min(indices)
+            return (
+                Format_Item(strip_string[:index].rstrip()),
+                Format_Item(strip_string[index:]),
+            )
+        return None
+
     def tostr(self):
         """
         :return: Parsed representation of two format items
diff --git a/src/fparser/two/tests/fortran2003/test_format_item_c1002.py b/src/fparser/two/tests/fortran2003/test_format_item_c1002.py
index 8d00209f..a49c5a25 100644
--- a/src/fparser/two/tests/fortran2003/test_format_item_c1002.py
+++ b/src/fparser/two/tests/fortran2003/test_format_item_c1002.py
@@ -42,6 +42,7 @@
 
 import pytest
 from fparser.two.Fortran2003 import Format_Item_C1002
+from fparser.two import utils
 from fparser.two.utils import InternalError, NoMatchError
 
 
@@ -106,3 +107,58 @@ def test_internal_error3(f2003_create, monkeypatch):
     assert (
         "items entry 1 should contain a format items object but it " "is empty or None"
     ) in str(excinfo.value)
+
+
+@pytest.mark.parametrize(
+    "my_input,expected",
+    [
+        ("'a' 1x", "'a', 1X"),
+        ("'a' 'b'", "'a', 'b'"),
+        ("'a''b' 'c'", "'a''b', 'c'"),
+        ("15x'a'", "15X, 'a'"),
+        ("21x ' if it is the case '", "21X, ' if it is the case '"),
+        ('1x"don\'t"', '1X, "don\'t"'),
+        ("'a' 1x 'b'", "'a', 1X, 'b'"),
+    ],
+)
+def test_format_missing_comma_extension(f2003_create, my_input, expected):
+    """Check that a missing comma between a character-string edit
+    descriptor and a neighbouring format item is matched when the
+    'format-missing-comma' extension is enabled (which it is by
+    default).
+
+    """
+    ast = Format_Item_C1002(my_input)
+    assert str(ast) == expected
+
+
+@pytest.mark.parametrize("my_input", ["'a' 1x", "15x'a'", "'a' 'b'"])
+def test_format_missing_comma_disabled(f2003_create, monkeypatch, my_input):
+    """Check that a missing comma between a character-string edit
+    descriptor and a neighbouring format item is not matched when the
+    'format-missing-comma' extension is not enabled.
+
+    """
+    monkeypatch.setattr(
+        utils,
+        "_EXTENSIONS",
+        [ext for ext in utils.EXTENSIONS() if ext != "format-missing-comma"],
+    )
+    with pytest.raises(NoMatchError):
+        _ = Format_Item_C1002(my_input)
+
+
+def test_format_missing_comma_format_statement(f2003_create):
+    """Check that format statements making use of the
+    'format-missing-comma' extension are parsed (see issue #484 and
+    the MODFLOW-2005 codebase).
+
+    """
+    from fparser.two.Fortran2003 import Format_Stmt
+
+    ast = Format_Stmt("FORMAT(//1X,'SWR PROCESS REQUIRES LAYCON'1X,'FOR',//)")
+    assert (
+        str(ast) == "FORMAT(/, /, 1X, 'SWR PROCESS REQUIRES LAYCON', 1X, 'FOR', /, /)"
+    )
+    ast = Format_Stmt("FORMAT(/1x,'unable to find:',1x,A,/15x' in file:')")
+    assert str(ast) == "FORMAT(/, 1X, 'unable to find:', 1X, A, /, 15X, ' in file:')"
diff --git a/src/fparser/two/tests/fortran2003/test_format_specification_r1002.py b/src/fparser/two/tests/fortran2003/test_format_specification_r1002.py
index 53e06e3a..4a2dd0e5 100644
--- a/src/fparser/two/tests/fortran2003/test_format_specification_r1002.py
+++ b/src/fparser/two/tests/fortran2003/test_format_specification_r1002.py
@@ -38,6 +38,7 @@
 """
 
 import pytest
+from fparser.two import utils
 from fparser.two.Fortran2003 import Format_Specification
 from fparser.two.utils import NoMatchError
 
@@ -247,7 +248,7 @@ def test_syntaxerror(f2003_create):
             _ = Format_Specification(my_input)
 
 
-def test_syntaxerror_c1002(f2003_create):
+def test_syntaxerror_c1002(f2003_create, monkeypatch):
     """Test that we get an exception in situations where no comma is
     supplied and the C1002 constraints for optional commas do not
     apply.
@@ -259,9 +260,25 @@ def test_syntaxerror_c1002(f2003_create):
     for my_input in ["(2P, 2/)", "('hello', 2/)", "(2E2.2, 3/)"]:
         ast = Format_Specification(my_input)
     # invalid syntax
-    for my_input in ["(2P 2/)", "(2P2/)", "('hello' 2/)", "('hello'2/)"]:
+    for my_input in ["(2P 2/)", "(2P2/)"]:
         with pytest.raises(NoMatchError):
             _ = Format_Specification(my_input)
+    # A missing comma after a character-string edit descriptor is
+    # accepted by the 'format-missing-comma' extension (enabled by
+    # default) ...
+    for my_input in ["('hello' 2/)", "('hello'2/)"]:
+        ast = Format_Specification(my_input)
+        assert str(ast) == "('hello', 2/)"
+    # ... and is invalid syntax without the extension.
+    with monkeypatch.context() as mpatch:
+        mpatch.setattr(
+            utils,
+            "_EXTENSIONS",
+            [ext for ext in utils.EXTENSIONS() if ext != "format-missing-comma"],
+        )
+        for my_input in ["('hello' 2/)", "('hello'2/)"]:
+            with pytest.raises(NoMatchError):
+                _ = Format_Specification(my_input)
     # Comma is mandatory after a P descriptor if not one of ['F', 'E',
     # 'EN', 'ES', 'D', 'G'] or not a '/' or a ':'
     # Test valid syntax.
@@ -285,7 +302,6 @@ def test_syntaxerror_c1002(f2003_create):
         _ = Format_Specification(my_input)
     # Comma is mandatory if C1002 is not relevant
     for my_input in [
-        "('hello' 'hello')",
         "(2P 2P)",
         "(2P2P)",
         "(F2.2 F2.2)",
@@ -299,3 +315,17 @@ def test_syntaxerror_c1002(f2003_create):
     ]:
         with pytest.raises(NoMatchError):
             _ = Format_Specification(my_input)
+    # A missing comma between two character-string edit descriptors is
+    # accepted by the 'format-missing-comma' extension (enabled by
+    # default) ...
+    ast = Format_Specification("('hello' 'hello')")
+    assert str(ast) == "('hello', 'hello')"
+    # ... and is invalid syntax without the extension.
+    with monkeypatch.context() as mpatch:
+        mpatch.setattr(
+            utils,
+            "_EXTENSIONS",
+            [ext for ext in utils.EXTENSIONS() if ext != "format-missing-comma"],
+        )
+        with pytest.raises(NoMatchError):
+            _ = Format_Specification("('hello' 'hello')")
diff --git a/src/fparser/two/utils.py b/src/fparser/two/utils.py
index b7985a80..37a03b95 100644
--- a/src/fparser/two/utils.py
+++ b/src/fparser/two/utils.py
@@ -118,6 +118,13 @@
 # With this extension, these statements will be allowed.
 _EXTENSIONS += ["extended-stop-args"]
 
+# Many compilers (e.g. gfortran, ifort, ifx) accept a missing comma
+# between a character-string edit descriptor and a neighbouring format
+# item in a format specification, e.g. FORMAT('a' 1x,'b'), FORMAT(15x'a')
+# or FORMAT('a' 'b'). This is supported by fparser if
+# 'format-missing-comma' is specified in the EXTENSIONS list.
+_EXTENSIONS += ["format-missing-comma"]
+
 
 def EXTENSIONS():
     """