The High Court has today ordered BAE Systems to produce a sworn affidavit divulging how they obtained a confidential and legally privileged document belonging to Campaign Against Arms Trade (CAAT).
The document in question contained advice from CAAT's solicitors on a planned judicial review of the decision to drop a corruption probe into BAE's dealings with Saudi Arabia.
The Serious Fraud Office (SFO) controversially dropped its investigation into the Saudi Arabia deal in December last year.
BAE faces scrutiny over corruption allegations in six nations.
CAAT spokesperson Symon Hill said: "We are delighted. This is a victory not only for Campaign Against Arms Trade but for all who care about democracy. When Tony Blair ended the Saudi corruption inquiry, he implied that BAE Systems were above the law. But today BAE have been prevented from behaving as they like. We are a step closer to the day when BAE can no longer get away with calling the shots."
CAAT suggested that judicial review proceedings could be severely prejudiced if BAE had access to CAAT's confidential legal advice. The campaigners say that BAE Systems failed to convince the court that they had no obligation to give away the source of their information.
BAE now have until 12th March to produce their affidavit.
CAAT and the Corner House - an anti-corruption and social justice group - will make an announcement shortly about their application for a judicial review.
The leaked CAAT information contained specifics regarding tactics and costs for the anti-arms group, including legal advice from CAAT's lawyers over its case regarding the Serious Fraud Office.
Following Monday's ruling, BAE said that once presented with the e-mail, the firm "immediately brought it to the attention of CAAT".
The judge said that even though BAE returned the e-mail, allowing the leaker to remain anonymous had "facilitated the continuation of the wrong" done to CAAT.
The move also opened the chance that the person could leak the same information to another party opposed to CAAT, said the judge.
The court order was needed as BAE and its legal representatives, Allen & Overy, had refused to provide information to reveal who had sent the e-mail, said Dinah Rose, QC, who has been acting for CAAT.
Campaign Against Arms Trade (CAAT) works for the reduction and ultimate abolition of the international arms trade.