Shadow chancellor Ed Balls has denied involvement in a plot to oust Tony Blair as Prime Minister, following the leak of a cache of private documents detailing Gordon Brown's preparations to take power.
The papers, obtained by the Daily Telegraph, show how Mr Balls and current Labour leader Ed Miliband were assigned roles by Mr Brown in an operation to ensure his succession as PM.
Whitehall officials are investigating whether the leak amounted to a breach of Government secrecy.
The papers - including handwritten memos from Mr Brown and letters from Mr Blair to his then chancellor - were last seen in Mr Balls's office at the Department for Children. They appear to have gone missing from boxes of private possessions passed on to his House of Commons office following Labour's election defeat in 2010.
Following a complaint from Mr Balls, the Cabinet Office said officials were looking into whether the papers had been "in the possession of any Government department", and if so, whether there had been "any breaches of document security within Government".
Mr Balls said: "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 dismissed as "false and mendacious" claims that the papers showed there had been a plot by Mr Brown's supporters to unseat Mr Blair, adding that: "The idea that there was a plot or a coup is untrue and not justified by these papers."
His role had been to try to "hold things together" at a turbulent time. Acknowledging that the relationship between Labour's most powerful figures "could have been better handled", he insisted that the party had changed since the days of "tensions and rows" between Blairites and Brownites.
"There are important lessons to learn, people want to know that the Labour Party has learned them," said Mr Balls. "We have, 100%."
Meanwhile, Mr Miliband vowed the current generation of Labour chiefs would not repeat the "mistakes" of Mr Blair and Mr Brown. But Conservative Party deputy chairman Michael Fallon said that the papers showed that Mr Balls and Mr Miliband could not be trusted.