Quinn Emanuel Urquhart & Sullivan has been ordered to reveal the identity of the supplier of an allegedly forged document that claimed that Russian oligarch Oleg Deripaska had grossly underpaid a rival, by some $300 million, in a buyout deal.

Handing down the judgment on Monday, London's High Court granted Deripaska what's known as 'Norwich pharmacal relief' order—a type of order that can compel a party to disclose information that can clear another of wrongdoing.