diff --git a/src/context.cpp b/src/context.cpp
index e2cffd1db1..46efc05d8e 100644
--- a/src/context.cpp
+++ b/src/context.cpp
@@ -373,6 +373,14 @@ namespace Sass {
     // process the resolved entry
     else if (resolved.size() == 1) {
       bool use_cache = c_importers.size() == 0;
+      if (resolved[0].deprecated) {
+        // emit deprecation warning when import resolves to a .css file
+        deprecated(
+          "Including .css files with @import is non-standard behaviour which will be removed in future versions of LibSass.",
+          "Use a custom importer to maintain this behaviour. Check your implementations documentation on how to create a custom importer.",
+          true, pstate
+        );
+      }
       // use cache for the resource loading
       if (use_cache && sheets.count(resolved[0].abs_path)) return resolved[0];
       // try to read the content of the resolved file entry
diff --git a/src/file.cpp b/src/file.cpp
index 826c8113ce..b40067649b 100644
--- a/src/file.cpp
+++ b/src/file.cpp
@@ -357,13 +357,13 @@ namespace Sass {
       for(auto ext : d_exts) {
         rel_path = join_paths(base, "_" + name + ext);
         abs_path = join_paths(root, rel_path);
-        if (file_exists(abs_path)) includes.push_back({{ rel_path, root }, abs_path });
+        if (file_exists(abs_path)) includes.push_back({{ rel_path, root }, abs_path, true });
       }
       // next test plain name with d_exts
       for(auto ext : d_exts) {
         rel_path = join_paths(base, name + ext);
         abs_path = join_paths(root, rel_path);
-        if (file_exists(abs_path)) includes.push_back({{ rel_path, root }, abs_path });
+        if (file_exists(abs_path)) includes.push_back({{ rel_path, root }, abs_path, true });
       }
       // index files
       if (includes.size() == 0) {
diff --git a/src/file.hpp b/src/file.hpp
index 759996fa85..718a529cd1 100644
--- a/src/file.hpp
+++ b/src/file.hpp
@@ -89,9 +89,14 @@ namespace Sass {
     public:
       // resolved absolute path
       std::string abs_path;
+      // is a deprecated file type
+      bool deprecated;
     public:
+      Include(const Importer& imp, std::string abs_path, bool deprecated)
+      : Importer(imp), abs_path(abs_path), deprecated(deprecated)
+      { }
       Include(const Importer& imp, std::string abs_path)
-      : Importer(imp), abs_path(abs_path)
+      : Importer(imp), abs_path(abs_path), deprecated(false)
       { }
   };