diff --git a/IPython/core/magics/extension.py b/IPython/core/magics/extension.py
index 65f3993258c..2991d55ca40 100644
--- a/IPython/core/magics/extension.py
+++ b/IPython/core/magics/extension.py
@@ -29,35 +29,6 @@
 class ExtensionMagics(Magics):
     """Magics to manage the IPython extensions system."""
 
-    @line_magic
-    def install_ext(self, parameter_s=''):
-        """Download and install an extension from a URL, e.g.::
-
-            %install_ext https://bitbucket.org/birkenfeld/ipython-physics/raw/d1310a2ab15d/physics.py
-
-        The URL should point to an importable Python module - either a .py file
-        or a .zip file.
-
-        Parameters:
-
-          -n filename : Specify a name for the file, rather than taking it from
-                        the URL.
-        """
-        warn("%install_ext` is deprecated, please distribute your extension "
-             "as a python package.", UserWarning)
-        opts, args = self.parse_options(parameter_s, 'n:')
-        try:
-            filename = self.shell.extension_manager.install_extension(args,
-                                                                 opts.get('n'))
-        except ValueError as e:
-            print(e)
-            return
-
-        filename = os.path.basename(filename)
-        print("Installed %s. To use it, type:" % filename)
-        print("  %%load_ext %s" % os.path.splitext(filename)[0])
-
-
     @line_magic
     def load_ext(self, module_str):
         """Load an IPython extension by its module name."""
diff --git a/IPython/core/tests/test_magic.py b/IPython/core/tests/test_magic.py
index a507ab9861d..9718d19e844 100644
--- a/IPython/core/tests/test_magic.py
+++ b/IPython/core/tests/test_magic.py
@@ -605,16 +605,13 @@ def test_prun_quotes():
     nt.assert_equal(_ip.user_ns['x'], '\t')
 
 def test_extension():
-    # Debugging information for failures of this test
-    print('sys.path:')
-    for p in sys.path:
-        print(' ', p)
-    print('CWD', os.getcwd())
-
-    nt.assert_raises(ImportError, _ip.magic, "load_ext daft_extension")
-    daft_path = os.path.join(os.path.dirname(__file__), "daft_extension")
-    sys.path.insert(0, daft_path)
+    tmpdir = TemporaryDirectory()
+    orig_ipython_dir = _ip.ipython_dir
     try:
+        _ip.ipython_dir = tmpdir.name
+        nt.assert_raises(ImportError, _ip.magic, "load_ext daft_extension")
+        url = os.path.join(os.path.dirname(__file__), "daft_extension.py")
+        _ip.magic("install_ext %s" % url)
         _ip.user_ns.pop('arq', None)
         invalidate_caches()   # Clear import caches
         _ip.magic("load_ext daft_extension")
@@ -622,8 +619,8 @@ def test_extension():
         _ip.magic("unload_ext daft_extension")
         assert 'arq' not in _ip.user_ns
     finally:
-        sys.path.remove(daft_path)
-
+        _ip.ipython_dir = orig_ipython_dir
+        tmpdir.cleanup()
 
 @dec.skip_without('nbformat')
 def test_notebook_export_json():
diff --git a/docs/source/whatsnew/pr/incompat-deleted-install_ext.rst b/docs/source/whatsnew/pr/incompat-deleted-install_ext.rst
new file mode 100644
index 00000000000..01b6d750497
--- /dev/null
+++ b/docs/source/whatsnew/pr/incompat-deleted-install_ext.rst
@@ -0,0 +1 @@
+Deleted the install_ext magic function