Shadow chancellor Ed Balls today 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 were today investigating whether the leak amounted to a breach of Government secrecy.
The papers - including hand-written 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.
"The idea that there was a plot or a coup is untrue and not justified by these papers," said Mr Balls.
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.
"Everything wasn't always perfect about their relationship," said Mr Miliband.
"We are not going to make those mistakes. Frankly this is ancient history - that era is over and we are looking forward to the future."
But Conservative Party deputy chairman Michael Fallon said that the papers showed that Mr Balls and Mr Miliband could not be trusted.
"While Britain's debt doubled, welfare spending spiralled out of control and education standards fell, they were obsessing about getting rid of the elected prime minister and putting Gordon Brown into position," said Mr Fallon.
"Instead of owning up to their role in a dysfunctional government and coming up with a credible plan to deal with the problems facing Britain, they are starting to plot against each other. They can never be trusted with government again."
Cabinet minister Michael Gove, who took over from Mr Balls at the renamed Department for Education last May, appeared confident that his officials were not to blame for the leak.
A source close to the Education Secretary said: "Like with (former Haringey social services chief) Sharon Shoesmith, Ed Balls is pathetically trying to blame officials.
"He should ask his best friend Damian McBride how these things get leaked."
The reference to Mr McBride, a former Brown aide thought to be behind negative briefings against rivals, suggests those around Mr Gove suspect the leak to be a result of internal Labour feuding.
The documents show that discussions about the Labour succession were under way within Mr Brown's inner circle just weeks after the party's 2005 election victory.
A lengthy memo from Mr Brown accuses Mr Blair of "self-promotion" and says the then PM was happy to see the media frame Labour's internal wrangling as a battle between himself as a reformist and the chancellor as a block on reform.
"If we are to renew Labour, we must be as rigorous and as brutal as we were in the creation of New Labour," wrote Mr Brown.
Memos gave Mr Balls the task of bringing together a "small group" of Brown loyalists - including Mr Miliband and current shadow foreign secretary Douglas Alexander - to prepare for a leadership election.
Mr Miliband was assigned the job of highlighting Mr Brown's "third world idealism, environmental action, community spirit and an ethos of public service" in order to "redefine politics from spin/calculation/manoeuvre".
Also revealed was a letter from Mr Blair to Mr Brown setting out his proposals for an orderly transfer of power, but demanding that the chancellor work with him to avoid a "breakdown" and warning: "The division at the top is killing us."
In his characteristic block-capital handwriting, Mr Brown scrawled "shallow, inconsistent, muddled" over the top of the note. And he replied with a statement, which he hoped Mr Blair would sign, effectively handing control over future policy to Mr Brown.
Mr Balls denied this amounted to evidence of a coup plot, telling BBC Radio 4's World at One: "There had to be planning, there had to be discussion ... there was not a plot, but there was genuine and open and sometimes difficult discussion."
And he was backed by Blairite former minister Lord (Jim) Knight, who told the BBC: "All these papers do is show that amongst the small group of people who had been working for Gordon, there was work done on the succession. It was important for the sake of the country and the Labour Party that the succession was done to the best possible effect."