Ed Balls denies 2005 plot to oust Blair as Whitehall investigates leak

• Relations between Tony Blair (left) and Gordon Brown became increasingly strained in the later years of the Labour Government

Whitehall officials today announced the move after the the cache of documents was obtained by the Daily Telegraph newspaper.

The leaked documents shed new light on the roles of Ed Miliband and Ed Balls in Mr Brown's plot to take over from Mr Blair as Prime Minister.

Current Labour leader Mr Miliband and shadow chancellor Mr Balls began working against the then prime minister in mid-July 2005 on the instruction of Mr Brown, leaked letters and documents claim.

They include private memos written by hand by Mr Brown, as well as letters from Mr Blair to his former Chancellor.

Reports today suggested that the documents came from the personal files of shadow chancellor Mr Balls and went missing after he moved out of the Department for Children following last year's general election.

It is understood that they were not among boxes of papers delivered to his House of Commons office by the department.

A Cabinet Office spokesman said today: "The Cabinet Office is looking first into whether this particular set of papers was in the possession of any Government department and, if so, whether there have been any breaches of document security within government."

Downing Street confirmed the inquiry had been launched in response to a request from Mr Balls. The shadow chancellor said he welcomed the investigation into the leak of the documents.

"The last time I saw them was when they were on my desk in the department. I don't know how they were taken and got to the Telegraph," he told the BBC.

He strongly denied claims the papers showed there had been a plot by Mr Brown's supporters, dismissing the suggestions as "false and mendacious".

"The idea that there was a plot or a coup is untrue and not justified by these papers," he told the BBC.