David Miliband has called for the Labour Party to unite behind his brother Ed, following press stories of a feud between the two.
A book serialised by The Mail on Sunday claims David is unhappy about Labour's direction under Ed's leadership and is barely on speaking terms with him.
Senior figures are said to fear a return to the infighting of the Blair/Brown years.
David has responded by saying he stands "fully behind" his younger brother.
In a statement, the former foreign secretary wrote: "I have moved on from the leadership election and so should everyone else.
"Ed won, I stand fully behind him and so should everyone else.
"I called for unity last October and I repeat that now.
"We all have our part to play in supporting Ed and the frontbench team to ensure we expose this government for its reckless policies that are damaging the country.
"The rest is soap opera of which I want no part and the public have no interest."
The statement comes as The Sunday Times was forced to admit it had wrongly attributed a quote criticising Ed Miliband's leadership to former Deputy Prime Minister Lord Prescott.
In a story entitled "Labour big beasts maul Ed Miliband", the Labour peer is quoted as saying: "He (Ed Miliband) picked up what Gordon did, which was write off the first 10 years of the Labour government, which were the good years.
"It is only early days, but it's not been a good start."
But Lord Prescott published a series of Tweets on Sunday morning claiming the quote was a "lie".
The Sunday Times admitted it had made an error, Tweeting: "Due to a prod error a quote was wrongly attributed to @johnprescott.
"We apologise for the confusion & are happy to set the record straight."
A source close to Lord Prescott said he had taken a call from the reporter who wrote the piece but had hung up straight away.
"John hates journalists and never talks to Sunday newspapers," the source added.
Former Lord Chancellor, Lord Falconer, told BBC News that press stories of feuds at the top of the Labour Party were "highly reminiscent" of the Blair/Brown era and were "damaging" to the party.
"It is trying to fit the present and the future into a template from the past; There is no doubt that Gordon and Tony got on incredibly badly," said the former Lord Chancellor.
"'Everybody' therefore wants that sort of rowing to continue and it's got the added spice of the fact that the two leading figures in the Labour Party are brothers - but that's not the way it is."
Ed and David were on different sides in the feud between Gordon Brown and Tony Blair during Labour's time in power.
Both have acknowledged the damage that row did to the party and the need to avoid it happening again.
David has largely remained silent on policy issues since his defeat and avoided public criticism of Ed's performance as leader.
But the newspapers have been dominated in recent days by leaked documents, including the speech David planned to make if he had won the leadership.
Some commentators see this as a concerted attempt by David's supporters to destabilise Ed's leadership.
Shadow health secretary Jon Healey denied reports of in-fighting between rival camps.
"There is no whispering or plots against Ed Miliband," he told BBC One's Politics Show.
He said Labour was in a "unique position" for a party that had just lost an election as there was "a determination and a unity that we simply have not seen before".
And shadow defence secretary Jim Murphy, writing on the grassroots Labour List website, said: "My advice to everyone involved in this week's briefing and counter-briefing is just shut up and get on with your job."
A new unauthorised biography, Ed: The Milibands And The Making Of A Labour Leader, claims David Miliband can barely bring himself to speak to his brother, and the two men communicate mainly through officials.
David is also said to be scathing about Ed's performance in private, supposedly saying he is "heading in the wrong direction".
Ed, for his part, is said to regard his sibling as too "managerial and technocratic".
The book also claims there is "bad blood" between Ed and shadow chancellor Ed Balls, dating back to their time as advisers to Gordon Brown at the Treasury.
The biography - by Labour-supporting journalists Mehdi Hasan and James Macintyre - questions Ed's claim he made a last-minute decision to stand, claiming he may have been plotting to eclipse David for years.
The book says Ed blames David's team for spreading his nickname Red Ed.
It adds that the two brothers have clashed over how Ed broke the news he was planning to stand.
The Labour leader will try to regain the initiative on Monday, with a speech in which he will admit Labour got it wrong over the welfare state and banking regulation.
A source close to the Labour leader said: "David and Ed talked before, during and after the leadership election.
"There is no problem. This is tittle tattle and the Labour Party will be concentrating on meeting the challenges of Britain's future, not looking back to the past."