By <a href="http://twitter.com/alex__stevenson">Alex Stevenson</a>
Ed Miliband faces serious questions about his leadership, even after a show of loyalty from his elder brother David.
Disquiet about his ability to take the fight to the coalition, despite a series of government U-turns, has prompted the first widespread debate about his future as leader of the opposition.
Several newspapers carried stories citing sources close to David Miliband indicating his willingness to challenge Ed Miliband. The Independent on Sunday said friends of the Blairite favourite believe David Miliband is "waiting for his brother Ed to fail".
But David Miliband issued a statement on Sunday insisting that he was not interested in undermining the Labour party's leader.
"I have moved on from the leadership election and so should everyone else," he said.
"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 front bench team to ensure we expose this government for its reckless polices that are damaging the country.
"The rest is soap opera of which I want no part and the public have no interest."
But a book serialised by the Mail on Sunday, written by two prominent left-wing journalists, reveals Mr Miliband's elder brother, whom he narrowly beat in last year's leadership election, as "poised" to mount a challenge should the opportunity arise.
The former foreign secretary effectively accuses Ed Miliband of lying over the so-called "act of fratricide".
Ed Miliband has claimed he told his brother about his intention to run for the Labour leadership face-to-face. David Miliband claims he first heard of the news in a television report.
Serious concerns about Ed Miliband's performance are common among MPs, peers and Labour party donors, the Sunday Times reported. Former home secretary David Blunkett said the next year would prove "vital in creating momentum and a sense of direction".
The newspaper quoted an anonymous Labour MP as suggesting Ed Miliband faced a threat from another quarter - shadow chancellor Ed Balls, who the MP claimed has a "cunning plan" to undermine Mr Miliband by making Mr Balls appear a team player.
"Balls will then be in pole position to replace Ed whenever the vacancy arises," the MP explained.
"Nobody's pretending there will be a vacancy any time soon. But Balls wants to be in place."
Tomorrow Mr Miliband will attempt to relaunch his leadership by making a speech on welfare reform, as legislation pushing through the coalition's plan to shake up the benefits system is debated in the Commons.
His remarks could inflame tensions within the party by criticising New Labour's record in government – a tactic which has alarmed many New Labour supporters.
"Labour, a party founded by hard-working people for hard-working people, was seen by some – however unfairly – as the party of those ripping off society," Mr Miliband is expected to say.
"A 'take what you can' culture which began in the 1980s was allowed to continue, unchecked, under the last government."
Those on the left of the Labour party could be alarmed when Mr Miliband goes further than Peter Mandelson, who famously remarked that he was "intensely relaxed" about people getting 'filthy rich'.
"I'm not only relaxed about them getting rich," Mr Miliband will say. "I applaud it."
A source close to Mr Miliband sought to downplay speculation about the Labour leader's future by dismissing it as "tittle-tattle".
"David and Ed talked before, during and after the leadership election," the source said. "There is no problem... the Labour party will be concentrating on meeting the challenges of Britain's future, not looking back to the past."
A spokesman for David Miliband said the former favourite to take over from Gordon Brown had "moved on".
"This is soap opera speculation about history when the public want politicians to be focusing on the future," he said.
Supporters of the younger Miliband brother were left pointing to his performance in the polls. 'Tweet4Labour' observed: "Ed Miliband has led Labour up 14% in the polls. If he did that every year until election same people would complain about one-party state."
A YouGov poll for the Sunday Times put Labour on 42%, five points ahead of the Conservatives on 37%. The Liberal Democrats attracted just nine per cent.
Over half of respondents said Mr Miliband was doing badly as Labour leader, however, compared to just 30% who thought he was doing well.
Two-thirds were unclear what he stands for. Forty-one per cent said they thought David Miliband would do a better job – and just six per cent thought he would do a worse one.