diff --git a/wagtail/admin/auth.py b/wagtail/admin/auth.py
index ffc8d35d7a..a549d7f661 100644
--- a/wagtail/admin/auth.py
+++ b/wagtail/admin/auth.py
@@ -35,7 +35,7 @@ def users_with_page_permission(page, permission_type, include_superusers=True):
 
 def permission_denied(request):
     """Return a standard 'permission denied' response"""
-    if request.is_ajax():
+    if request.headers.get('x-requested-with') == 'XMLHttpRequest':
         raise PermissionDenied
 
     from wagtail.admin import messages
@@ -141,7 +141,7 @@ def user_has_any_page_permission(user):
 
 
 def reject_request(request):
-    if request.is_ajax():
+    if request.headers.get('x-requested-with') == 'XMLHttpRequest':
         raise PermissionDenied
 
     # import redirect_to_login here to avoid circular imports on model files that import
@@ -193,12 +193,12 @@ def require_admin_access(view_func):
                     return view_func(request, *args, **kwargs)
 
             except PermissionDenied:
-                if request.is_ajax():
+                if request.headers.get('x-requested-with') == 'XMLHttpRequest':
                     raise
 
                 return permission_denied(request)
 
-        if not request.is_ajax():
+        if not request.headers.get('x-requested-with') == 'XMLHttpRequest':
             messages.error(request, _('You do not have permission to access the admin'))
 
         return reject_request(request)
diff --git a/wagtail/admin/views/pages/search.py b/wagtail/admin/views/pages/search.py
index 9d8d05f986..eb6e02c2ac 100644
--- a/wagtail/admin/views/pages/search.py
+++ b/wagtail/admin/views/pages/search.py
@@ -93,7 +93,7 @@ def search(request):
     paginator = Paginator(pages, per_page=20)
     pages = paginator.get_page(request.GET.get('p'))
 
-    if request.is_ajax():
+    if request.headers.get('x-requested-with') == 'XMLHttpRequest':
         return TemplateResponse(request, "wagtailadmin/pages/search_results.html", {
             'pages': pages,
             'all_pages': all_pages,
diff --git a/wagtail/admin/views/pages/workflow.py b/wagtail/admin/views/pages/workflow.py
index 0223cdcde8..6c8faff11e 100644
--- a/wagtail/admin/views/pages/workflow.py
+++ b/wagtail/admin/views/pages/workflow.py
@@ -88,13 +88,13 @@ class WorkflowAction(BaseWorkflowFormView):
             form = self.form_class(request.POST)
             if form.is_valid():
                 redirect_to = self.task.on_action(self.task_state, request.user, self.action_name, **form.cleaned_data) or self.redirect_to
-            elif self.action_modal and request.is_ajax():
+            elif self.action_modal and request.headers.get('x-requested-with') == 'XMLHttpRequest':
                 # show form errors
                 return self.render_modal_form(request, form)
         else:
             redirect_to = self.task.on_action(self.task_state, request.user, self.action_name) or self.redirect_to
 
-        if request.is_ajax():
+        if request.headers.get('x-requested-with') == 'XMLHttpRequest':
             return render_modal_workflow(request, '', None, {}, json_data={'step': 'success', 'redirect': redirect_to})
         return redirect(redirect_to)
 
@@ -113,7 +113,7 @@ class CollectWorkflowActionData(BaseWorkflowFormView):
         form = self.form_class(request.POST)
         if form.is_valid():
             return render_modal_workflow(request, '', None, {}, json_data={'step': 'success', 'cleaned_data': form.cleaned_data})
-        elif self.action_modal and request.is_ajax():
+        elif self.action_modal and request.headers.get('x-requested-with') == 'XMLHttpRequest':
             # show form errors
             return self.render_modal_form(request, form)
 
diff --git a/wagtail/contrib/redirects/views.py b/wagtail/contrib/redirects/views.py
index 0c765fa1f4..8502bfb130 100644
--- a/wagtail/contrib/redirects/views.py
+++ b/wagtail/contrib/redirects/views.py
@@ -54,7 +54,7 @@ def index(request):
     redirects = paginator.get_page(request.GET.get('p'))
 
     # Render template
-    if request.is_ajax():
+    if request.headers.get('x-requested-with') == 'XMLHttpRequest':
         return TemplateResponse(request, "wagtailredirects/results.html", {
             'ordering': ordering,
             'redirects': redirects,
diff --git a/wagtail/contrib/search_promotions/views.py b/wagtail/contrib/search_promotions/views.py
index 48be21a0c4..664a5ba53a 100644
--- a/wagtail/contrib/search_promotions/views.py
+++ b/wagtail/contrib/search_promotions/views.py
@@ -48,7 +48,7 @@ def index(request):
     paginator = Paginator(queries, per_page=20)
     queries = paginator.get_page(request.GET.get('p'))
 
-    if request.is_ajax():
+    if request.headers.get('x-requested-with') == 'XMLHttpRequest':
         return TemplateResponse(request, "wagtailsearchpromotions/results.html", {
             'is_searching': is_searching,
             'ordering': ordering,
diff --git a/wagtail/core/models/__init__.py b/wagtail/core/models/__init__.py
index 62fbce294e..d9ce1d07a0 100644
--- a/wagtail/core/models/__init__.py
+++ b/wagtail/core/models/__init__.py
@@ -1102,7 +1102,7 @@ class Page(AbstractPage, index.Indexed, ClusterableModel, metaclass=PageBase):
         return context
 
     def get_template(self, request, *args, **kwargs):
-        if request.is_ajax():
+        if request.headers.get('x-requested-with') == 'XMLHttpRequest':
             return self.ajax_template or self.template
         else:
             return self.template
diff --git a/wagtail/users/views/groups.py b/wagtail/users/views/groups.py
index d28b1914ff..414184082b 100644
--- a/wagtail/users/views/groups.py
+++ b/wagtail/users/views/groups.py
@@ -61,7 +61,7 @@ class IndexView(mixins.SearchableListMixin, generic.IndexView):
     ordering = ['name']
 
     def get_template_names(self):
-        if self.request.is_ajax():
+        if self.request.headers.get('x-requested-with') == 'XMLHttpRequest':
             return ['wagtailusers/groups/results.html']
         else:
             return ['wagtailusers/groups/index.html']
diff --git a/wagtail/users/views/users.py b/wagtail/users/views/users.py
index bde0411f1d..37bb1bf51d 100644
--- a/wagtail/users/views/users.py
+++ b/wagtail/users/views/users.py
@@ -101,7 +101,7 @@ def index(request, *args):
     paginator = Paginator(users.select_related('wagtail_userprofile'), per_page=20)
     users = paginator.get_page(request.GET.get('p'))
 
-    if request.is_ajax():
+    if request.headers.get('x-requested-with') == 'XMLHttpRequest':
         return TemplateResponse(request, "wagtailusers/users/results.html", {
             'users': users,
             'is_searching': is_searching,