Jonathan Powell, Tony Blair's chief of staff, once said that Gordon Brown was like a character in a Shakespearean tragedy because he would never achieve his ambition to become Prime Minister. It could be worse than that. He could end up as the Labour leader who reached No 10 and failed, forced out before even having the chance to fight a general election.

We will know within days - possibly within hours - whether Mr Brown will survive or whether he will end up as what Roy Jenkins once described damningly as a “tail-end Charlie” PM, hanging on to the coat tails of Mr Blair.

Already Mr Brown's authority seems to be slipping fatally away. Last night, James Purnell joined Hazel Blears in walking out of the Cabinet. The reshuffle, normally the Prime Minister's moment of maximum power, is exposing his growing weakness. The number of backbenchers who have joined the e-mail campaign calling for him to go is mounting. It is the final act of a psychodrama that has been acted out at Westminster for 15 years.

This is a Shakespearean tragedy, rather than a Greek one, because the hero will be brought down by a fatal character flaw and not by fate. Labour MPs love to debate which tragic hero Mr Brown is most like. Some say he has Hamlet's tendency to dither - agonising whether “to be or not to be” in favour of public service reform. To others it is Macbeth's “vaulting ambition which o'er-leaps itself” that will bring him down. For years he was consumed by Othello's “green-eyed monster”, a destructive jealousy of Mr Blair. This week he more resembles King Lear, driven to distraction by the perceived ingratitude of his daughters who rages on the heath that he is “more sinned against than sinning”.

The danger for the Labour Party is that the play is in fact Romeo and Juliet, a fight to the death between two tribes - Blairite Capulets and Brownite Montagues - that will end in the annihilation of them all.

This is a leadership crisis for Mr Brown, but it is no coincidence that the spirit of Mr Blair still stalks the stage like Banquo's ghost. It is too simplistic to say that it's a Blairite plot - although Hazel Blears and Mr Purnell are both from that wing of the party. Lord Mandelson, Tony's consigliere has now become Gordon's comforter, pleading with Labour MPs not to betray the man he once turned against himself.

But new Labour was created by the combination of Mr Blair and Mr Brown and the party is still convulsed by the TB-GBs. Until recently, the Prime Minister even had two strategy meetings - a “Blairite” one, attended by Lord Mandelson, Alastair Campbell and Philip Gould and a “Brownite” one with Ed Balls and Charlie Whelan. He has two people writing Labour's next manifesto: a Blairite, Patrick Diamond, and a Brownite, Ed Miliband. Even reshuffle speculation has been haunted by the previous Prime Minister. Mr Balls would be a controversial choice as chancellor because he did so much to undermine party unity by briefing against Mr Blair when he was working for Mr Brown.

Since John Smith died in 1994, and the younger man snatched the crown from his more experienced friend, the characters and fortunes of the two politicians have been inextricably entwined. They still speak almost every week by phone. Although Mr Blair once told a friend that Mr Brown could not beat David Cameron - “unless he changes, and he cannot change” - the relationship has always been more complicated than it is often portrayed. On the weekend after the so-called curry coup by the Brownites, two former Cabinet ministers were planning to declare Mr Brown unfit to be leader: I am told that Mr Blair telephoned them to tell them not to do so because it would split the party.

The relationship has not always been so positive. Just as Gordon scuppered much of Tony's premiership by trying to block changes to schools and hospitals so now Tony hangs over Gordon's time in charge. Mr Brown has struggled to assert his own identity in No 10 because he spent his first few months trying to prove he was not Mr Blair. During his years at the Treasury he put himself on the wrong (left wing) side of his own dividing line with No 10 making it harder to appeal to Middle England now that he is Prime Minister. As a result of his frustration with Mr Blair, he also honed the style - the bunker mentality, the smear tactics, the demands for loyalty - that have now alienated so many of his own MPs.

“The die was cast at Granita,” one minister said yesterday. “The Labour Party was engaged in a pact that was undemocratic and we are paying the price. It's not about ideology, it's about personality. Labour will lose power because we are showing our inner workings in a way that's incredibly unattractive.”

In the early days, it was Mr Blair's ability to “emote” that made him seem a modern leader in the age of Diana, Big Brother and GMTV. But it is also emotion that has for years threatened to destroy a party dominated by dysfunctional relationships. It's not just Mr Brown who has psychological flaws. “It's government by hissy fit,” a minister once protested to me. As new Labour tried to put ideology behind it, personality filled the vacuum. Having suppressed its instincts and tolerated Mr Blair, a leader it never loved, in order to win power, the party is no longer sure what to believe now that it is led by Mr Brown. The Prime Minister has not helped the identity crisis. “He's morally bankrupt,” a Cabinet minister said after the Damian McBride resignation. “If you think you can't win the argument on substance you end up falling back on political fixes and smears.”

Blood will have blood and the tragedy shows no sign of ending. If anything the animosity between the younger members of Labour's two tribes is more intense than that between Mr Brown and Mr Blair. The leaders had a love-hate relationship; their followers have been left with only the hate. After Tony and Gordon have exited stage left, the jostling for position will only intensify. “The TB-GBs will live on in the shape of Balls versus Purnell, or Balls versus Miliband,” says a minister. “The real tragedy is that this may not in fact be the last act of the psychodrama.”

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