diff --git a/changelog/778.removal.rst b/changelog/778.removal.rst
new file mode 100644
index 000000000..0b21f77f3
--- /dev/null
+++ b/changelog/778.removal.rst
@@ -0,0 +1 @@
+The ability to create an `.NDCollection` object with numerical keys is deprecated as it leads to ambiguous behavior when slicing the collection.
diff --git a/ndcube/ndcollection.py b/ndcube/ndcollection.py
index 0b37e4e91..30790b5d1 100644
--- a/ndcube/ndcollection.py
+++ b/ndcube/ndcollection.py
@@ -1,9 +1,11 @@
+import numbers
 import textwrap
 import collections.abc
 
 import numpy as np
 
 import ndcube.utils.collection as collection_utils
+from ndcube.utils.exceptions import warn_deprecated
 
 __all__ = ["NDCollection"]
 
@@ -14,7 +16,7 @@ class NDCollection(dict):
 
     Parameters
     ----------
-    data: sequence of `tuple` of (`str`, `~ndcube.NDCube` or `~ndcube.NDCubeSequence`)
+    key_data_pairs: sequence of `tuple` of (`str`, `~ndcube.NDCube` or `~ndcube.NDCubeSequence`)
         The names and data cubes/sequences to held in the collection.
 
     aligned_axes: `tuple` of `int`, `tuple` of `tuple` of `int`, 'all', or None, optional
@@ -47,6 +49,12 @@ class NDCollection(dict):
     """
 
     def __init__(self, key_data_pairs, aligned_axes=None, meta=None, **kwargs):
+        for key, _ in key_data_pairs:
+            if isinstance(key, numbers.Number):
+                warn_deprecated(
+                    "Passing numerical keys to NDCollection is deprecated as they"
+                    " lead to ambiguity when slicing the collection."
+                )
         # Enter data and metadata into object.
         super().__init__(key_data_pairs)
         self.meta = meta