In /Config/UrlRewriting.config you can add:

<add name="enRewrite"
    virtualUrl="~/en/?(.*)"
    rewriteUrlParameter="ExcludeFromClientQueryString"
    destinationUrl="~/$1"
    redirectMode="Permanent"
    redirect="Application"
    ignoreCase="true" />

If you are using IIS7+ you can use Url Rewrite Module - http://www.iis.net/learn/extensions/url-rewrite-module/url-rewrite-module-configuration-reference

In web.config you can add:

<system.webServer>
    <rewrite> 
      <rules>
        <rule name="enRewrite">
          <match url="/en/?(.+)" />
          <action type="Redirect" url="{R:1}" appendQueryString="false" />
        </rule>                 
      </rules>
    </rewrite>
</system.webServer>

UrlRewriting.config addition worked but the rule for the web.config didn't work as expected. Any further advice would be appreciated as the rewrite will be needed long term and UrlRewriting.config is becoming deprecated in Umbraco 8.

The URL Rewriting rule also create errors when ever a "en" appears in a link. Removing the tilde allowed it to work.

<add name="enRewrite"
    virtualUrl="/en/(.*)"
    rewriteUrlParameter="ExcludeFromClientQueryString"
    destinationUrl="~/$1"
    redirectMode="Permanent"
    redirect="Application"
    ignoreCase="true" />

If you dont want to go with UrlRewriting.config then I have testet this one with success:

<rewrite> 
  <rules>
    <rule name="enRewrite" stopProcessing="true">
      <match url="^en(.+)?" />
      <action type="Redirect" url="{R:1}" appendQueryString="false" />
    </rule>                 
  </rules>
</rewrite>

If it does not work, then try to enable failed request tracing and see if you can see the error: http://www.iis.net/learn/extensions/url-rewrite-module/using-failed-request-tracing-to-trace-rewrite-rules

BTW - if you are not running IIS express then you can use the IIS Manager -> Url Rewrite tool to test your rules.

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