diff --git a/CHANGES.rst b/CHANGES.rst
index 37083327..89a68018 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -2,6 +2,24 @@ Changelog
 =========
 
 
+0.18.2 (not yet released)
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Bug fixes and minor changes
+---------------------------
+
++ `#83`_, `#84`_: enable ordering on one to many relationships in
+  class :class:`icat.query.Query`.
+
++ `#84`_: Add warning classes
+  :exc:`icat.exception.QueryOneToManyOrderWarning` and
+  :exc:`icat.exception.QueryWarning`, the latter being a common base
+  class for warnings emitted during creation of a query.
+
+.. _#83: https://github.com/icatproject/python-icat/issues/83
+.. _#84: https://github.com/icatproject/python-icat/pull/84
+
+
 0.18.1 (2021-04-13)
 ~~~~~~~~~~~~~~~~~~~
 
diff --git a/doc/src/exception.rst b/doc/src/exception.rst
index b36cb28a..1378cad1 100644
--- a/doc/src/exception.rst
+++ b/doc/src/exception.rst
@@ -103,10 +103,18 @@ Exceptions raised by python-icat
     :members:
     :show-inheritance:
 
+.. autoexception:: icat.exception.QueryWarning
+    :members:
+    :show-inheritance:
+
 .. autoexception:: icat.exception.QueryNullableOrderWarning
     :members:
     :show-inheritance:
 
+.. autoexception:: icat.exception.QueryOneToManyOrderWarning
+    :members:
+    :show-inheritance:
+
 .. autoexception:: icat.exception.ClientVersionWarning
     :members:
     :show-inheritance:
@@ -178,7 +186,9 @@ The class hierarchy for the exceptions is::
    +-- IDSResponseError
    +-- GenealogyError
    +-- Warning
-        +-- QueryNullableOrderWarning
+        +-- QueryWarning
+        |    +-- QueryNullableOrderWarning
+        |    +-- QueryOneToManyOrderWarning
         +-- ClientVersionWarning
         +-- DeprecationWarning
              +-- ICATDeprecationWarning
diff --git a/icat/exception.py b/icat/exception.py
index 876f2c65..df37e8be 100644
--- a/icat/exception.py
+++ b/icat/exception.py
@@ -26,7 +26,7 @@
     # icat.config
     'ConfigError', 
     # icat.query
-    'QueryNullableOrderWarning', 
+    'QueryWarning', 'QueryNullableOrderWarning', 'QueryOneToManyOrderWarning',
     # icat.client, icat.entity
     'ClientVersionWarning', 'ICATDeprecationWarning', 
     'EntityTypeError', 'VersionMethodError', 'SearchResultError', 
@@ -305,14 +305,34 @@ class ConfigError(_BaseException):
 
 # ================ Exceptions raised in icat.query =================
 
-class QueryNullableOrderWarning(Warning):
-    """Warn about using a nullable relation for ordering.
+class QueryWarning(Warning):
+    """Warning while building a query.
+
+    .. versionadded:: 0.18.2
+    """
+    pass
+
+class QueryNullableOrderWarning(QueryWarning):
+    """Warn about using a nullable many to one relation for ordering.
+
+    .. versionchanged:: 0.18.2
+        Inherit from :exc:`QueryWarning`.
     """
     def __init__(self, attr):
-        msg = ("ordering on a nullable relation implicitly "
+        msg = ("ordering on a nullable many to one relation implicitly "
                "adds a '%s IS NOT NULL' condition." % attr)
         super(QueryNullableOrderWarning, self).__init__(msg)
 
+class QueryOneToManyOrderWarning(QueryWarning):
+    """Warn about using a one to many relation for ordering.
+
+    .. versionadded:: 0.18.2
+    """
+    def __init__(self, attr):
+        msg = ("ordering on a one to many relation %s may surprisingly "
+               "affect the search result." % attr)
+        super().__init__(msg)
+
 
 # ======== Exceptions raised in icat.client and icat.entity ========
 
diff --git a/icat/query.py b/icat/query.py
index c2ad4d10..47bbf24e 100644
--- a/icat/query.py
+++ b/icat/query.py
@@ -262,8 +262,12 @@ def setOrder(self, order):
             name and an order direction, the latter being either "ASC"
             or "DESC" for ascending or descending order respectively.
         :type order: iterable or :class:`bool`
-        :raise ValueError: if `order` contains invalid attributes that
-            either do not exist or contain one to many relationships.
+        :raise ValueError: if any attribute in `order` is not valid.
+
+        .. versionchanged:: 0.18.2
+            allow one to many relationships in `order`.  Emit a
+            :exc:`~icat.exception.QueryOneToManyOrderWarning` rather
+            then raising a :exc:`ValueError` in this case.
         """
         if order is True:
 
@@ -291,9 +295,9 @@ def setOrder(self, order):
                             warn(QueryNullableOrderWarning(pattr), 
                                  stacklevel=sl)
                     elif attrInfo.relType == "MANY":
-                        raise ValueError("Cannot use one to many relationship "
-                                         "in '%s' to order %s." 
-                                         % (obj, self.entity.BeanName))
+                        sl = 3 if self._init else 2
+                        warn(QueryOneToManyOrderWarning(pattr),
+                             stacklevel=sl)
 
                 if rclass is None:
                     # obj is an attribute, use it right away.
diff --git a/tests/test_06_query.py b/tests/test_06_query.py
index 08c378bb..43ca3754 100644
--- a/tests/test_06_query.py
+++ b/tests/test_06_query.py
@@ -6,6 +6,7 @@
 from distutils.version import StrictVersion as Version
 import re
 import sys
+import warnings
 import pytest
 import icat
 import icat.config
@@ -316,6 +317,33 @@ def test_query_nullable_warning_suppressed(client, recwarn):
     res = client.search(query)
     assert len(res) == 44
 
+def test_query_order_one_to_many(client, recwarn):
+    """Sort on a related object in a one yo many relation.
+    This has been enabled in #84, but a warning is still emitted.
+    """
+    recwarn.clear()
+    query = Query(client, "Investigation",
+                  order=['investigationInstruments.instrument.fullName'])
+    w = recwarn.pop(icat.QueryOneToManyOrderWarning)
+    assert issubclass(w.category, icat.QueryOneToManyOrderWarning)
+    assert "investigationInstruments" in str(w.message)
+    print(str(query))
+    res = client.search(query)
+    assert len(res) == 3
+
+def test_query_order_suppress_warnings(client, recwarn):
+    """Suppress all QueryWarnings.
+    """
+    recwarn.clear()
+    with warnings.catch_warnings():
+        warnings.simplefilter("ignore", category=icat.QueryWarning)
+        query = Query(client, "Investigation",
+                      order=['investigationInstruments.instrument.fullName'])
+    assert len(recwarn.list) == 0
+    print(str(query))
+    res = client.search(query)
+    assert len(res) == 3
+
 def test_query_limit(client):
     """Add a LIMIT clause to the last example.
     """