A US judge has allowed Oracle to use an internal Google email in its Java patent infringement lawsuit against Android.

The email, which Google was fighting to keep Oracle from submitting, is a critical piece of evidence that could make things go horribly wrong for Google.

Oracle had filed a patent infringement lawsuit against Google last year, alleging that its Android mobile operating system has copied heaps of Java source code without adequate license.

The email includes text from a Google engineer claiming that the company needed to negotiate a license for using Java in Android. According to the email, Google then went on to look for Java alternatives for Android, but failed to find anything as good as Java.

Google wanted to keep Oracle from using the email, claiming that it was protected by the attorney-client privilege.

According to Reuters, Judge William Alsup ruled the email was not protected under the privilege clauses and could be used by Oracle against Google.

“Google has failed to identify any aspect of the challenged order that was clearly erroneous or contrary to law," wrote Alsup in his ruling.