The Week in Politics: Eight years on, the blood brothers are heading for a bloodbath

"I know what you want to talk about," the cabinet minister said when he returned my call. "No you don't," I replied. "The relationship," he said. He was right: the civil war between Tony Blair and Gordon Brown obsesses Westminster as much as the prospect of war in Iraq.

That this once-strong political marriage has turned sour is now beyond doubt. The two architects of New Labour have had plenty of ups and downs since Blair became party leader in 1994. One of the strengths of the partnership was that they had their rows in private without the bruises showing in public. Last autumn, something changed. Now both men bear the scars and there is little attempt to cover them up.

I still find it hard to believe. I have known Blair and Brown since the days when they shared a Commons office too small to swing a copy of Hansard in. At the time, their partnership was so strong I could not imagine it ever being broken. When the London Evening Standard ran an article predicting the two friends would one day be rivals, I laughed and told everyone they would never fall out. I had not yet learnt that politics is such a rough trade. Perhaps I should have seen their split coming. In 1994, the junior partner inherited the Labour crown when John Smith died. The senior partner's grief turned to grievance.

Until then, I had regarded myself as being on the same friendly terms with Blair and Brown. When Blair emerged as the modernisers' most likely standard-bearer in the leadership election, Brown appeared to believe – wrongly – that journalists including me were part of a plot to deny him the candidacy. He button-holed me in a Commons corridor and, referring to my employer at the time, growled: "When this is all over, I want you to explain to me how all the Murdoch papers came out for Tony so soon after John died." From that point, in Brown's eyes, I was foe rather than friend. "If you are not 100 per cent with Gordon, he thinks you are 100 per cent against him," a cabinet minister explained.

Why has the Blair-Brown relationship become dysfunctional? My hunch is that the biggest reason is simply the passage of time. Brown has now been waiting his turn for eight and a half years. Since the 2001 general election, he has asked Blair on more than one occasion when he intends to stand down. No doubt the Chancellor fears his strong economic record may become tarnished by global events well beyond his control. As in 1994, when Blair sparkled as shadow Home Secretary and Brown's duty was to be a dour shadow Chancellor, he may again be in the wrong place when the music stops.

The two men always had different instincts: Blair, scarred by Labour's 18 years in the wilderness, is desperate to keep his umbilical link with Middle Britain. Brown's heart, in contrast, lies with traditional Labour.

The Chancellor's impatience to land the glittering prize has made it harder for him to conceal these differences. Although they agree on the need to reform public services, they diverge sharply on how to do it. Blair is pushing hard a "choice and diversity" agenda that rewards the best performers with more cash and freedom, in the hope that standards will be "levelled up". Brown is wary of injecting market forces into public services and wonders how making the best hospitals even better will help the people who rely on the worst ones.

The Prime Minister drove through a similar plan to bring the market into higher education. Hence his attraction to allowing universities to charge variable top-up fees and students paying back their individual fees, while Brown's more centralist instincts pointed him to a flat-rate graduate tax.

It was no coincidence that Brown clashed very publicly with Alan Milburn, the Health Secretary, over foundation hospitals and Charles Clarke, the Education Secretary, on top-up fees. Blair sided against Brown in both battles.

The Chancellor got a draw on hospitals but lost on universities. Milburn and Clarke are both ambitious and seen by Blairites who do not want Brown to land the crown as a possible "Stop Gordon" candidate. When David Blunkett was their flavour of the month, he clashed with Brown over Home Office funding. All roads lead back to the succession; Brown never rows with his cabinet allies.

Another source of tension between the Prime Minister and Chancellor is the euro. Brown, it seems, has rebuffed Blair's attempts even to discuss it. Brown wants to rule out a referendum until after the next general election, while Blair is desperate to keep open the option.

According to some ministers, the euro is the issue that could finally break the relationship. Blairites float the once-unimaginable scenario of Brown being ousted from the Treasury if he vetoes a referendum. Some claim Blair is so fed up with Brown that he no longer wants him to succeed him. If true, that would be a travesty. Brownites hit back with talk of their man becoming a "nuclear weapon" sitting on the Labour back benches.

We are approaching the climax of this drama. As one minister told me: "We can't go on like this. There is corrosion at the heart of government. One of them will not be in the same job by this time next year." Whether they like it or not, the one-time blood brothers are still tied together. It is in both their interests to prevent an economic slide and deliver some progress on public services.

If they divorce, it will be the end of New Labour and possibly the Labour Government. The Brownites think Blair wants another five years in the job, and are probably right. So the Chancellor will have to learn to be patient if he is to get the chance he deserves.

There is still time to avoid a tragic ending. But not much.