diff --git a/WHATSNEW b/WHATSNEW
index 80d14ea23..8658a1933 100644
--- a/WHATSNEW
+++ b/WHATSNEW
@@ -25,6 +25,10 @@ Fixed bugs:
    setting a proxy caused problems with other Java libraries.
    Bugzilla Report 50888.
 
+ * Include task breaks dependencies or extension-points for multiple
+   files.
+   Bugzilla Report 50866.
+
 Other changes:
 --------------
 
diff --git a/src/main/org/apache/tools/ant/taskdefs/ImportTask.java b/src/main/org/apache/tools/ant/taskdefs/ImportTask.java
index 26d7a43ac..50487f6f4 100644
--- a/src/main/org/apache/tools/ant/taskdefs/ImportTask.java
+++ b/src/main/org/apache/tools/ant/taskdefs/ImportTask.java
@@ -206,10 +206,14 @@ public class ImportTask extends Task {
         boolean oldIncludeMode = ProjectHelper.isInIncludeMode();
         String oldSep = ProjectHelper.getCurrentPrefixSeparator();
         try {
-            String prefix = targetPrefix;
+            String prefix;
             if (isInIncludeMode() && oldPrefix != null
                 && targetPrefix != null) {
                 prefix = oldPrefix + oldSep + targetPrefix;
+            } else if (targetPrefix != null) {
+                prefix = targetPrefix;
+            } else {
+                prefix = oldPrefix;
             }
             setProjectHelperProps(prefix, prefixSeparator,
                                   isInIncludeMode());
diff --git a/src/tests/antunit/bugfixes/br50866/br50866-test.xml b/src/tests/antunit/bugfixes/br50866/br50866-test.xml
new file mode 100644
index 000000000..9e2d20375
--- /dev/null
+++ b/src/tests/antunit/bugfixes/br50866/br50866-test.xml
@@ -0,0 +1,13 @@
+<project name="br50866" default="antunit" xmlns:au="antlib:org.apache.ant.antunit">
+
+    <import file="../../antunit-base.xml" />
+
+    <include file="middle.xml" />
+
+    <target name="testinclude" depends="middle.common,middle.middle">
+        <au:assertPropertyEquals name="prop-common-init" value="br50866"/>
+        <au:assertPropertyEquals name="prop-common" value="br50866"/>
+        <au:assertPropertyEquals name="prop-middle" value="br50866"/>
+    </target>
+
+</project>
\ No newline at end of file
diff --git a/src/tests/antunit/bugfixes/br50866/common.xml b/src/tests/antunit/bugfixes/br50866/common.xml
new file mode 100644
index 000000000..7417c0de9
--- /dev/null
+++ b/src/tests/antunit/bugfixes/br50866/common.xml
@@ -0,0 +1,8 @@
+<project name="common" default="default">
+    <target name="common-init">
+        <property name="prop-common-init" value="br50866" />
+    </target>
+    <target name="common" depends="common-init">
+        <property name="prop-common" value="br50866" />
+    </target>
+</project>
\ No newline at end of file
diff --git a/src/tests/antunit/bugfixes/br50866/middle.xml b/src/tests/antunit/bugfixes/br50866/middle.xml
new file mode 100644
index 000000000..b387f9315
--- /dev/null
+++ b/src/tests/antunit/bugfixes/br50866/middle.xml
@@ -0,0 +1,7 @@
+<project name="middle" default="default">
+    <import file="common.xml" optional="true" />
+
+    <target name="middle" depends="common-init">
+        <property name="prop-middle" value="br50866" />
+    </target>
+</project>
\ No newline at end of file