diff --git a/src/main/java/net/sourceforge/plantuml/servlet/PlantUmlServlet.java b/src/main/java/net/sourceforge/plantuml/servlet/PlantUmlServlet.java
index 6f2ae11..625cc9e 100644
--- a/src/main/java/net/sourceforge/plantuml/servlet/PlantUmlServlet.java
+++ b/src/main/java/net/sourceforge/plantuml/servlet/PlantUmlServlet.java
@@ -66,6 +66,9 @@ public class PlantUmlServlet extends HttpServlet {
     private static final Pattern RECOVER_UML_PATTERN = Pattern.compile("/uml/(.*)");
     static {
         OptionFlags.ALLOW_INCLUDE = false;
+        if ("true".equalsIgnoreCase(System.getenv("ALLOW_PLANTUML_INCLUDE"))) {
+            OptionFlags.ALLOW_INCLUDE = true;
+        }
     }
 
     @Override