If just one piece of evidence were needed to illustrate how bad relations between David and Ed Miliband became during Labour's first leadership contest in 16 years, it came on 13 July 2010 when the two brothers arrived with their aides at the TUC's summer reception at Church House in Westminster.

Onlookers recall the elder Miliband looking confident, self-assured, a leader-in-waiting. His entourage was equally focused. Whenever the younger Miliband struck up a conversation with a union leader, one of David's supporters would walk over and hover nearby - or even try to interrupt the two men. "David's team seemed to be in military mode," says a union man who was present. When David started a similar conversation, his supporters would form a protective ring around him to prevent any interruptions. "It might have been a coincidence," a witness remembers. "But I've never seen so many coincidences in the space of 25 minutes." "It was fucking outrageous," a senior member of Ed's campaign team says. "David's people behaved like a tag team."

As my co-author, James Macintyre, and I report in our new biography of Ed Miliband, such antics only heightened tensions between the two men and their camps. As the weeks passed, the distance between the brothers increased. A close family friend told us of how he spoke to the younger Miliband and asked him, bluntly: "Are you guys still talking to one another?" Ed told the friend that David was "trying very hard to be nice to him but he felt it was forced".

To claim the contest between the brothers was anything other than bitter and divisive, as some spinners on behalf of David and Ed have done in the past week, is nonsense. The "Ed Speaks Human" placards, which first appeared at the launch of the younger Miliband's leadership campaign at a Fabian Society event on 15 May, personally offended and annoyed David. Ed's aides maintain that these signs were not co-ordinated by them but one told us that "of course, the line was an attack on David". A close friend of Ed's described it as "hardball". Meanwhile, Ed was subjected to a whispering campaign. He was referred to, under cover of ano­nymity, as a "Bennite", "Red Ed" and "Forrest Gump". By August, says an aide, the regular attacks on Ed in the press from unnamed David supporters had unsettled and upset him.

Nearly a year on, Ed tends to tell friends that his relationship with David is "on the mend". It isn't. The two cannot even agree on the events surrounding Ed's decision to stand a year ago. It came as a surprise to discover that the two brothers and their camps had versions of the same event that were totally at odds with each other. Ed says he went to David's home in Primrose Hill, north London, on the evening of 12 May - the day his elder brother declared his candidacy for the Labour leadership - to inform him of his own decision to stand. In a story that Ed has since repeated to friends and in interviews, he says David was polite and understanding. "I'd rather you didn't run," David is said to have remarked. "I'd rather have a campaign where my brother was supporting me, if I'm really honest." But, he then added: "I don't want me to be the reason you don't stand, so I think you should do it."

Or did he? Today, neither David nor Ed can agree on when or even if this crucial meeting occurred. David is emphatic there was no such meeting: his younger brother did not set foot in his house that week. Ed, for his part, is adamant he had the conversation with, and broke the news to, David in the latter's home on that Wednesday evening, six days after the general election.

Given their failure to align their stories, or agree a common line for public consumption, what seems at first glance to be a trivial discrepancy is, in fact, a demonstration of the distrust that now exists between the brothers. Aside from the odd phone call, the two men now tend to communicate through each others' aides and advisers. It is an oddly formal and distant relationship for brothers who shared a house as adults and served in cabinet together - the first brothers to do so since 1938, when Edward and Oliver Stanley, sons of the 17th Earl of Derby, both served on Neville Chamberlain's front bench.

An even bigger shock was to discover that the political has also become personal: Ed and David's family lives have been infected by the fallout from the divisive leadership contest. Some friends of the Milibands had hoped that their young children would unite them. Yet, in December, when David and his wife Louise hosted a sixth birthday party at their home for their elder son, Isaac, there was no sign of Ed, his wife Justine, and their two sons, Daniel and Samuel. No one knows whether they were invited and declined, or were not asked. It remains a mystery. Meanwhile, the regular Sunday lunches where the two brothers would gather with their families and their mother, Marion, are now a dim memory.

Unspoken argument

Does any of this matter? Is it all ancient history, as spokesmen for both brothers tried to suggest on 12 June - Ed's "Bloody Sunday" in the words of the New Statesman blogger Dan Hodges, as revelations from our book began to appear in the newspapers - or is it a key part of understanding who Ed Miliband is and how his leadership will unfold? Ed's relationship with David matters because the elder Miliband is considered by his supporters and admirers - both in parliament and in the media - as Labour's "lost leader", the "king across the water". He has told close friends that he believes Labour is heading in the wrong direction under Ed's leadership. Indeed, David's very presence, some would say, is a threat to his brother's grip on a demoralised and unruly Labour Party.

On the afternoon of Sunday 12 June, as rumours of a leadership plot swirled around Westminster, David released a statement on his personal website. "I have moved on from the leadership election and so should everyone else," the elder Miliband said. "Ed won, I stand fully behind him and so should everyone else." He added: "The rest is soap opera of which I want no part and the public have no interest."

At around the same time, a close friend and ally of the former foreign secretary rang to tell me that "David is very keen for everyone to recognise that he is fully behind Ed and not plotting against him." Yet only a day earlier, the day before our biography was serialised, the Guardian front-page headline read: "The speech David Miliband would have given - if he'd won". Where did the copy of the speech come from? A source at the Guardian suggests that it was a member of David's inner circle, with the consent of the former foreign secretary, who passed on the text to the newspaper's political correspondents.

David's allies deny the charge. They point out that the elder Miliband had his victory speech uploaded on to the conference hall autocues ahead of the result being announced in Manchester, so that he could practise delivering it. It was, therefore, available to a wider circle of people than David's campaign team.

Regardless, the impact of the leak was to further destabilise Ed Miliband's leadership - and invite commentators to draw unhelpful comparisons between his own speech and the speech that David never gave. (It is worth noting, however, that Ed's conference speech on 28 September distanced Labour from the authoritarianism and militarism of the recent past in a way that David would never have been able to.) Some have argued in recent days that David has every right to "stab Ed in the back", on the grounds that it was Ed who betrayed David to begin with by daring to stand against his elder brother. The unspoken argument is that younger brothers defer to elder brothers; age trumps ability. (The Labour MP Jon Cruddas was so annoyed that Ed had dared challenge his own older brother that he is said to have told David during last year's leadership campaign: "Why don't you fucking punch him? That's what I'd do." A startled David said nothing in response.)

The situation now is very different. David was not the leader when Ed challenged him: the two brothers ran against each in an open, democratic race to fill a vacancy at the top of the party. Memories are conveniently short, and what David's allies are doing now - perhaps with the encouragement of David himself - is undermining the elected Labour leader. "It feels very ugly," a senior Labour peer and ally
of Ed Miliband tells me now. "Ed needs to be aggressive and forceful in his own defence." The peer adds: "There needs to be a sense that the Labour Party and the shadow cabinet want Ed Miliband as their leader."

Nevertheless the fallout from the book's revelations, and the Guardian splash, were handled badly by Team Ed. Why was it left to Charles Falconer, the former lord chancellor and close ally of David - who admittedly has since become an informal adviser to the younger Miliband - to come out in defence of the Labour leader on the BBC ?

“The responsibility lies with the shadow cabinet," says a former Labour cabinet minister. "When they were the victim of 'plot' and 'coup' rumours, Tony and Gordon would always use the trick of sending four or five cabinet heavyweights on to the airwaves to shut the story down. If I were Ed, my eyes would be swivelling to Douglas Alexander, Yvette Cooper and Caroline Flint. Why haven't they come out to defend him?"

Instead, deadlines - some arbitrary, some silly - for Ed Miliband have been set by pundits and politicians alike: he has until his speech at the party conference in Liverpool in September; he has until the London mayoral elections in May 2012; he has until the Olympics in the summer of 2012. Meanwhile the line that he has yet to define himself or a direction of travel for his party has been so overused it has become clichéd. Few journalists or columnists bother to read, let alone attend, Ed Miliband's speeches - at the Fabian Society, at the Resolution Foundation, at the Progress think tank - where he has outlined his views on intergenerational opportunity, income inequality, resilient communities and the "squeezed middle". A fundamental problem is our obsession with changing party leaders: it has resulted in the three main parties going through ten party leaders (including caretakers) in the past six years. Frankly, it is absurd to suggest that a Labour leader who has been in post for less than a year, and is ahead in the polls, should be dumped for no specific reason.

Two Milibands in a row

“Personally, Ed is very strong and he can cope with all this nonsense," says a friend. "Politically, however, he cannot carry on like this, with non-stop negative headlines and stories. If it carries on like this, he'll be gone within two years." But who is the alternative to Ed Miliband? Where is the replacement leader, waiting in the wings? This is not the Nineties or Noughties; there is no Gordon Brown figure lurking in the shadows. The shadow chancellor, Ed Balls, has not yet completed his detoxification strategy - and the recent leaked memos in the Daily Telegraph about his role in the so-called Brownite plots against Tony Blair were a stark reminder of just how much baggage he carries. Friends say Balls's own electoral fate is now tied to that of the "other Ed". The shadow foreign secretary, Yvette Cooper, meanwhile, defers to her husband, Balls, on the leadership. And, despite being tipped by Tony Blair in his recent memoir as a future Labour leader, the shadow foreign secretary, Douglas Alexander, lacks support across the party.

Then there is David Miliband - the great white hope for some on the right of the party. In an interview with the north-east newspaper the Journal, in December 2010, the former foreign secretary refused to rule himself out of running for the Labour leadership in the future: "I've got to admit I wish the leadership campaign had gone differently, but who knows what will happen in the future?" However, it is difficult to believe that the Labour Party would reject one Miliband in order to replace him with another. The public would be appalled; the media would have a field day.
As the leadership race reached its climax late last summer, Ed Miliband bumped into a friend on the terrace of the House of Commons who asked him why he was challenging his own brother. "I assume it's because you can't have two Milibands in a row," said the friend. "Yes, that's exactly it," replied Ed without hesitation.

He is right. The next leader of the Labour Party, whoever he or she happens to be, won't be called Miliband. David's moment came and went. It is time for David-ites to accept that Labour chose the younger, not the elder, brother as its leader.

“Ed: the Milibands and the Making of a Labour Leader" by Mehdi Hasan and James Macintyre is published by Biteback (£16.99)