diff --git a/lib/govuk_tech_docs.rb b/lib/govuk_tech_docs.rb
index cd19219f..fd454d52 100644
--- a/lib/govuk_tech_docs.rb
+++ b/lib/govuk_tech_docs.rb
@@ -22,6 +22,7 @@
 require "govuk_tech_docs/unique_identifier_generator"
 require "govuk_tech_docs/warning_text_extension"
 require "govuk_tech_docs/api_reference/api_reference_extension"
+require "govuk_tech_docs/http_prefix_extension"
 
 module GovukTechDocs
   # Configure the tech docs template
@@ -65,6 +66,7 @@ def self.configure(context, options = {})
     context.activate :unique_identifier
     context.activate :warning_text
     context.activate :api_reference
+    context.activate :sass_http_prefix
 
     context.helpers do
       include GovukTechDocs::TableOfContents::Helpers
diff --git a/lib/govuk_tech_docs/http_prefix_extension.rb b/lib/govuk_tech_docs/http_prefix_extension.rb
new file mode 100644
index 00000000..82dc427f
--- /dev/null
+++ b/lib/govuk_tech_docs/http_prefix_extension.rb
@@ -0,0 +1,21 @@
+module GovukTechDocs
+  class SassHttpPrefixExtension < Middleman::Extension
+    def after_configuration
+      app.extensions[:sprockets].environment.context_class.class_eval do
+        # Override the `asset-path()` helper available in Sprockets to
+        # return a directory rather than a file if the path ends with `/`
+        alias_method :orig_asset_path, :asset_path
+
+        def asset_path path, options = {}
+          if options.empty? && path.end_with?("/")
+            File.join(*[app.config[:http_prefix], path].compact)
+          else
+            orig_asset_path path, options
+          end
+        end
+      end
+    end
+  end
+end
+
+::Middleman::Extensions.register(:sass_http_prefix, GovukTechDocs::SassHttpPrefixExtension)