diff --git a/etc/spack/defaults/modules.yaml b/etc/spack/defaults/modules.yaml
index d53e05b2c8..75ec366117 100644
--- a/etc/spack/defaults/modules.yaml
+++ b/etc/spack/defaults/modules.yaml
@@ -40,9 +40,8 @@ modules:
     roots:
      tcl:    $spack/share/spack/modules
      lmod:   $spack/share/spack/lmod
-    # What type of modules to use
-    enable:
-      - tcl
+    # What type of modules to use ("tcl" and/or "lmod")
+    enable: []
 
     tcl:
       all:
diff --git a/lib/spack/spack/hooks/module_file_generation.py b/lib/spack/spack/hooks/module_file_generation.py
index e5e83cc9c8..dc86c43205 100644
--- a/lib/spack/spack/hooks/module_file_generation.py
+++ b/lib/spack/spack/hooks/module_file_generation.py
@@ -3,31 +3,24 @@
 #
 # SPDX-License-Identifier: (Apache-2.0 OR MIT)
 
-import llnl.util.tty as tty
+from llnl.util import tty
 
 import spack.config
 import spack.modules
-import spack.modules.common
 
 
 def _for_each_enabled(spec, method_name, explicit=None):
     """Calls a method for each enabled module"""
+    spack.modules.ensure_modules_are_enabled_or_warn()
     set_names = set(spack.config.get("modules", {}).keys())
-    # If we have old-style modules enabled, we put those in the default set
-    old_default_enabled = spack.config.get("modules:enable")
-    if old_default_enabled:
-        set_names.add("default")
     for name in set_names:
         enabled = spack.config.get("modules:%s:enable" % name)
-        if name == "default":
-            # combine enabled modules from default and old format
-            enabled = spack.config.merge_yaml(old_default_enabled, enabled)
         if not enabled:
             tty.debug("NO MODULE WRITTEN: list of enabled module files is empty")
             continue
 
-        for type in enabled:
-            generator = spack.modules.module_types[type](spec, name, explicit)
+        for module_type in enabled:
+            generator = spack.modules.module_types[module_type](spec, name, explicit)
             try:
                 getattr(generator, method_name)()
             except RuntimeError as e:
diff --git a/lib/spack/spack/modules/__init__.py b/lib/spack/spack/modules/__init__.py
index 4c56929a05..e9049a43bb 100644
--- a/lib/spack/spack/modules/__init__.py
+++ b/lib/spack/spack/modules/__init__.py
@@ -9,10 +9,15 @@
 
 from __future__ import absolute_import
 
-from .common import disable_modules
+from .common import disable_modules, ensure_modules_are_enabled_or_warn
 from .lmod import LmodModulefileWriter
 from .tcl import TclModulefileWriter
 
-__all__ = ["TclModulefileWriter", "LmodModulefileWriter", "disable_modules"]
+__all__ = [
+    "TclModulefileWriter",
+    "LmodModulefileWriter",
+    "disable_modules",
+    "ensure_modules_are_enabled_or_warn",
+]
 
 module_types = {"tcl": TclModulefileWriter, "lmod": LmodModulefileWriter}
diff --git a/lib/spack/spack/modules/common.py b/lib/spack/spack/modules/common.py
index b5e084bc0c..7dbe6d8988 100644
--- a/lib/spack/spack/modules/common.py
+++ b/lib/spack/spack/modules/common.py
@@ -33,7 +33,9 @@
 import datetime
 import inspect
 import os.path
+import pathlib
 import re
+import warnings
 from typing import Optional
 
 import llnl.util.filesystem
@@ -801,6 +803,43 @@ def verbose(self):
         return self.conf.verbose
 
 
+def ensure_modules_are_enabled_or_warn():
+    """Ensures that, if a custom configuration file is found with custom configuration for the
+    default tcl module set, then tcl module file generation is enabled. Otherwise, a warning
+    is emitted.
+    """
+
+    # TODO (v0.21 - Remove this function)
+    # Check if TCL module generation is enabled, return early if it is
+    enabled = spack.config.get("modules:default:enable", [])
+    if "tcl" in enabled:
+        return
+
+    # Check if we have custom TCL module sections
+    for scope in spack.config.config.file_scopes:
+        # Skip default configuration
+        if scope.name.startswith("default"):
+            continue
+
+        data = spack.config.get("modules:default:tcl", scope=scope.name)
+        if data:
+            config_file = pathlib.Path(scope.path)
+            if not scope.name.startswith("env"):
+                config_file = config_file / "modules.yaml"
+            break
+    else:
+        return
+
+    # If we are here we have a custom "modules" section in "config_file"
+    msg = (
+        f"detected custom TCL modules configuration in {config_file}, while TCL module file "
+        f"generation for the default module set is disabled. "
+        f"In Spack v0.20 module file generation has been disabled by default. To enable "
+        f"it run:\n\n\t$ spack config add 'modules:default:enable:[tcl]'\n"
+    )
+    warnings.warn(msg)
+
+
 class BaseModuleFileWriter(object):
     def __init__(self, spec, module_set_name, explicit=None):
         self.spec = spec