Two perceptions of the Miliband brothers hold sway in the Westminster rookery. One is that David is a prime minister in waiting while Ed, his younger sibling, lives in his shadow. The other is more intriguing – that David lacks the necessary skills for great office but Ed possesses them in abundance and will one day get his shot at No 10.

The contrasts between the Milibands – the first brothers to sit in cabinet since 1938 – are being fed into a Cain and Abel scenario. Ed may have to choose whether he backs his mentor Gordon Brown or his Blairite brother, who was accused last week of disloyally signalling his leadership intentions in a Guardian article.

Despite their age difference – David is 43 and Ed 38 – they are often confused with each other. At Ed’s first conference as a minister, a trade union leader apologised to him profusely: “David, David! I’m really sorry I called you Ed.” Sharing boyish looks, luxuriant dark hair and a cerebral manner, they have both enjoyed meteoric rises from the launchpad of a political dynasty. Their father, Ralph Miliband, the Marxist political theorist, was one of the most influential left-wingers of his generation.

The brothers Miliband went to the same comprehensive, studied at the same Oxford college and served for years at the feet of their warring masters, Tony Blair and Brown. They both live in Primrose Hill, north London – Ed with his girlfriend Justine, an environmental lawyer, and David with his wife Louise Shackle-ton, a violinist with the London Symphony Orchestra, and their two adopted children.

Their differences are more sharply observed at Westminster. David, who faces potential demotion from his job as foreign secretary in an autumn reshuffle, is unpopular with the parliamentary Labour party and has a reputation for being rude and arrogant. “He sweeps into the Commons with his Foreign Office advisers and doesn’t speak to backbench MPs,” said one lobby journalist. “There’s none of the tearoom banter which is the stuff of Labour politics.”

Even his friends concede that David is not a compelling speaker and has not shaken off his policy wonk image, while averring that in private he is affable and charming. “He’s not a natural back slapper,” said one.

Foreign Office mandarins may fawn on him for restoring their morale and standing after what they see as Margaret Beckett’s disastrous tenure, but Brownite critics say that he does not do “earth speak” and has stabbed his leader in the back.

Ed, by contrast, is regarded as a natural conciliator who could have been a relationship counsellor if he was not minister for the Cabinet Office. His easy, warm manner has won friends on both sides of the Blairite-Brown divide. It was Ed, during the fraught transition talks between Brown and Blair, who was welcomed into No 10 as Brown’s emissary in preference to the detested Ed Balls. He was dubbed “the ambassador from Planet F***” because he was the only one of the Brown team who refrained from telling the Blairites to f*** off.

“Ed is a brilliant communicator and is seen as a very friendly guy,” said another Westminster correspondent. “He’s the younger brother who has had to work harder socially and play the peacemaker. He combines brains with charm and political acumen. If he was older he would be seen as an electable leader.”

Unlike his elder sibling, Ed has worked in the real world, albeit briefly, as a television current affairs journalist. According to a former colleague: “There was never any doubt that he was going to go on and run the country in some way.” Mind you, Blair thought the same of David, whom he referred to as his Wayne Rooney. The assumption that the brothers were the heirs to Blair and Brown was apparent at Labour’s conference in 2005, when delegates sported “My favourite Miliband” badges, bearing pictures of either David or Ed.

Given that the brothers have belonged in opposing camps throughout their careers, it is perhaps surprising that they deny any rivalry. “We’re probably closer now than we’ve ever been,” David said.

Divorcing politics from family life was an early necessity in the Milibands’ argumentative household. Their father, a Polish Jew who had fled the German invasion of Belgium in 1940, encouraged the boys to enter debate with such dinner guests as Tony Benn, Tariq Ali and Ken Livingstone. Their mother, Marion Kozak, was also a left-wing intellectual.

David began leafleting for Labour at the age of nine and by their teens the boys were fully fledged campaigners. After his O-levels in 1986, Ed spent his summer holiday working for Benn. “Very helpful,” the left-wing veteran noted in his diaries. David later researched for Livingstone, who gave him a Sony Walkman.

The disparities of achievement between middle-class and working-class children at the boys’ north London comprehensive, Haver-stock, influenced their outlook. Although serious and single-minded, David scored poorly in his A-levels – “Ed did better than I did,” he admitted – but won a place at Oxford, where he got a first in politics, philosophy and economics. Although David exuded intellectual maturity, there was thought to be something unworldly about the way he composed essays in the college bar, holding a Mars bar and a pint of orange juice. After a year as a graduate student in America, he spent five years at the Institute for Public Policy Research, the think tank beloved of Labour modernisers.

In 1994, two months after his father’s death and within days of Blair being elected Labour leader, David was asked to become Blair’s head of policy. He went on to help to write the script for new Labour.

Ed followed him to Corpus Christi College and was caught up in student activism. “Politics motivated me more than academia,” he recalled. Other sibling differences were apparent: Ed was more angular, with a long bony face, and more cautious with his opinions. As a journalist he met Harriet Harman, then a shadow minister, who hired him as a speechwriter and researcher. Noticing his number-crunch-ing skills, Brown “burgled him off Harriet”, said Charlie Whelan, Brown’s spin doctor.

The brothers’ political views had long since diverged from their father’s ideological preoccupations towards a more pragmatic stance. Of the two, Ed has “more of his dad in him than David”, a former colleague said. Ralph’s pride in his sons was mingled with perplexity at the course they had set upon. David acknowledged: “I think he would think we are doing it for the right reasons. But I think he would have some pretty serious discussions about whether we were doing the right thing.”

In 2001 David was rewarded with the safe seat of South Shields which, he said, “opened my eyes, changed the way I think”. In a year he was catapulted to the post of schools minister. In 2004 he was promoted to Cabinet Office minister, replacing Ruth Kelly, with whom he had had a youthful fling. Last year he became the youngest foreign secretary since David Owen in 1977 – Brown’s reward for not standing against him.

Ed’s political rise was even faster. Swapping his job as Brown’s adviser for the Doncaster North constituency in 2005, he entered government as minister for the third sector a year later and joined the cabinet last year.

According to one theory, David’s recent action has made things easier for Ed. Last year David ducked out of standing against Brown and he will not be given another chance if he stays his hand a second time, leaving the field open to Ed at a later date. Alternatively, a leadership challenge by David soon might avert a fratricidal struggle for the top job in the future.

David has reportedly told his brother that if he becomes prime minister, he will judge Ed on his merits for any appointment. This arrangement took a knock last week when a poll suggested Labour’s poor support would decline under David’s premiership. Ed’s dilemma is compounded by the fact that Brown has made him co-ordinator of Labour’s next election manifesto. Last week a friend was quoted as saying that Ed was “completely loyal to Gor-don” and that if it came to a choice, “he would stick with Gordon”. But if Brown falls, Ed may have to campaign on David’s behalf.

The attention paid to the siblings bemuses some older hands. Linda McDougall, the journalist and author who is married to Austin Mitchell, the Labour MP, said recently: “The trouble is, I can’t take either of the Miliband brothers seriously enough. I used to change both their nappies.”