diff --git a/README.md b/README.md
index e4aafa72..f806e017 100644
--- a/README.md
+++ b/README.md
@@ -204,6 +204,7 @@ To sign xml documents:
         - `prefix` - adds this value as a prefix for the generated signature tags
         - `attrs` - a hash of attributes and values `attrName: value` to add to the signature root node
         - `location` - customize the location of the signature, pass an object with a `reference` key which should contain a XPath expression to a reference node, an `action` key which should contain one of the following values: `append`, `prepend`, `before`, `after`
+        - `existingPrefixes` - A hash of prefixes and namespaces `prefix: namespace` that shouldn't be in the signature because they already exist in the xml 
 - `getSignedXml()` - returns the original xml document with the signature in it, **must be called only after `computeSignature`**
 - `getSignatureXml()` - returns just the signature part, **must be called only after `computeSignature`**
 - `getOriginalXmlWithIds()` - returns the original xml with Id attributes added on relevant elements (required for validation), **must be called only after `computeSignature`**
diff --git a/lib/signed-xml.js b/lib/signed-xml.js
index 08d828bb..bd14cfc6 100644
--- a/lib/signed-xml.js
+++ b/lib/signed-xml.js
@@ -661,6 +661,7 @@ SignedXml.prototype.addReference = function(xpath, transforms, digestAlgorithm,
  * - `prefix` {String} Adds a prefix for the generated signature tags
  * - `attrs` {Object} A hash of attributes and values `attrName: value` to add to the signature root node
  * - `location` {{ reference: String, action: String }}
+ * - `existingPrefixes` {Object} A hash of prefixes and namespaces `prefix: namespace` already in the xml
  *   An object with a `reference` key which should
  *   contain a XPath expression, an `action` key which
  *   should contain one of the following values:
@@ -682,6 +683,7 @@ SignedXml.prototype.computeSignature = function(xml, opts) {
   prefix = opts.prefix;
   attrs = opts.attrs || {};
   location = opts.location || {};
+  existingPrefixes = opts.existingPrefixes || {};
   // defaults to the root node
   location.reference = location.reference || "/*";
   // defaults to append action
@@ -719,7 +721,16 @@ SignedXml.prototype.computeSignature = function(xml, opts) {
 
   this.originalXmlWithIds = doc.toString()
 
-  var signatureDoc = new Dom().parseFromString(this.signatureXml)
+  var existingPrefixesString = ""
+  Object.keys(existingPrefixes).forEach(function(key) {
+    existingPrefixesString += "xmlns:" + key + '="' + existingPrefixes[key] + '" '
+  });
+
+  // A trick to remove the namespaces that already exist in the xml
+  // This only works if the prefix and namespace match with those in te xml
+  var dummySignatureWrapper = "<Dummy " + existingPrefixesString + ">" + this.signatureXml + "</Dummy>"
+  var xml = new Dom().parseFromString(dummySignatureWrapper)
+  var signatureDoc = xml.documentElement.firstChild;
 
   var referenceNode = xpath.select(location.reference, doc);
 
@@ -730,13 +741,13 @@ SignedXml.prototype.computeSignature = function(xml, opts) {
   referenceNode = referenceNode[0];
 
   if (location.action === "append") {
-    referenceNode.appendChild(signatureDoc.documentElement);
+    referenceNode.appendChild(signatureDoc);
   } else if (location.action === "prepend") {
-    referenceNode.insertBefore(signatureDoc.documentElement, referenceNode.firstChild);
+    referenceNode.insertBefore(signatureDoc, referenceNode.firstChild);
   } else if (location.action === "before") {
-    referenceNode.parentNode.insertBefore(signatureDoc.documentElement, referenceNode);
+    referenceNode.parentNode.insertBefore(signatureDoc, referenceNode);
   } else if (location.action === "after") {
-    referenceNode.parentNode.insertBefore(signatureDoc.documentElement, referenceNode.nextSibling);
+    referenceNode.parentNode.insertBefore(signatureDoc, referenceNode.nextSibling);
   }
 
   this.signedXml = doc.toString()
diff --git a/test/signature-unit-tests.js b/test/signature-unit-tests.js
index dff41c29..0a7dc343 100644
--- a/test/signature-unit-tests.js
+++ b/test/signature-unit-tests.js
@@ -539,6 +539,49 @@ module.exports = {
         test.ok(!(err instanceof TypeError));
     }
     test.done();
+  },
+
+  "signer adds existing prefixes": function(test) {
+    function AssertionKeyInfo(assertionId) {
+      this.getKeyInfo = function(key, prefix) {
+        return '<wsse:SecurityTokenReference wsse11:TokenType="http://docs.oasis-open.org/wss/oasis-wss-saml-token-profile-1.1#SAMLV1.1" wsu:Id="0" ' +
+              'xmlns:wsse11="http://docs.oasis-open.org/wss/oasis-wss-wssecurity-secext-1.1.xsd"> ' +
+              '<wsse:KeyIdentifier ValueType="http://docs.oasis-open.org/wss/oasis-wss-saml-token-profile-1.0#SAMLAssertionID">'+assertionId+'</wsse:KeyIdentifier>'
+          '</wsse:SecurityTokenReference>';
+      };
+    }
+
+    var xml = 
+      '<SOAP-ENV:Envelope xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/"> ' +
+        '<SOAP-ENV:Header> ' +
+          '<wsse:Security ' +
+            'xmlns:wsse="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-secext-1.0.xsd" ' +
+            'xmlns:wsu="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-utility-1.0.xsd"> ' +
+            '<Assertion></Assertion> '+
+          '</wsse:Security> '+
+        '</SOAP-ENV:Header> '+
+      '</SOAP-ENV:Envelope>'
+
+    var sig = new SignedXml();
+    sig.keyInfoProvider = new AssertionKeyInfo(
+      "_81d5fba5c807be9e9cf60c58566349b1"
+    );
+    sig.signingKey = fs.readFileSync("./test/static/client.pem");
+    sig.computeSignature(xml, {
+      prefix: "ds",
+      location: {
+        reference: "//Assertion",
+        action: "after"
+      },
+      existingPrefixes: {
+        wsse: "http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-secext-1.0.xsd",
+        wsu: "http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-utility-1.0.xsd"
+      }
+    });
+    result = sig.getSignedXml();
+    test.equal((result.match(/xmlns:wsu=/g) || []).length, 1)
+    test.equal((result.match(/xmlns:wsse=/g) || []).length, 1)
+    test.done();
   }
 
 }