Yahoo wins battle over Prism court papers

The NSA's Prism surveillance system has prompted protests around the world

Details of the official justification for the US National Security Agency's wide-ranging Prism surveillance programme look set to be revealed.

Yahoo has won a legal fight that will see papers from a key 2008 court case declassified and published.

The 2008 case is widely seen as pivotal in letting the NSA establish Prism and start gathering data on web use.

The US government has been given until 29 July to say how long it will need to prepare the documents for publication.

Earlier this month, Yahoo filed papers with the Foreign Intelligence Surveillance Court (Fisc), seeking permission to publicise the documents it had filed in the original case and the government's response. The Fisc decides whether official applications to carry out surveillance should go ahead.

Yahoo took the legal action to show how vehemently it had objected to government requests to hand over data.

In addition, it said, the transcript of the 2008 case would reveal more about how the US government had justified its wide-ranging surveillance plan known as Prism.

In a statement, Yahoo said the release of the documents would "contribute constructively to the ongoing public discussion around online privacy".

Details about Prism were revealed by former NSA contractor Edward Snowden, who has now fled the US.

The US government has not filed any objections to the plan to disclose the court documents but will review the papers before publication so it can redact information it does not want published.

"The administration has said they want a debate about the propriety of the surveillance, but they haven't really provided information to inform that debate," Mark Rumold, a lawyer at the Electronic Frontier Foundation rights group, said.

"So declassifying these opinions is a very important place to start."