Andrew Grice: Deep potholes appear on Gordon's long and winding road to No 10

Gordon Brown is now at a crossroads both politically and economically. The road he hoped would take him to 10 Downing Street has proved much longer and more winding than he expected when he stood aside to give Tony Blair a clear run for the Labour leadership after John Smith's death in 1994.

Now the route is strewn with more potholes. Although you would not have known it from his upbeat presentation of the pre-Budget report yesterday, Mr Brown will have to keep his fingers crossed that his optimistic scenario for the British economy next year comes to fruition.

The biggest fear among the band of Brownites, who hope their man will succeed Mr Blair, is that if his economic record becomes tarnished, then his political prospects will be damaged too.

There are two faces of Mr Brown. There is The Charmer, who is capable of self-deprecating wit and who can build bridges with foes, as he has been doing with David Blunkett and Charles Clarke in recent weeks.

Then there is Grumpy Gordon, who is becoming increasingly impatient with his next-door neighbour. The Brownites claim their man had a gentleman's agreement with Mr Blair to take over the Number 10 tenancy halfway through this Parliament.

That falls in June next year – coincidentally, perhaps, the deadline for the completion of the Treasury's five economic tests on whether Britain should join the single currency.

The Brownites are now linking the two dates. There is increasing talk of a classic trade-off: Mr Blair could have the euro referendum he craves, but would first have to agree to stand down before the next general election.

Mr Brown's trump card is an effective veto on the euro; Mr Blair cannot risk a referendum without the Chancellor's 100 per cent support.

There is, however, a problem. Mr Blair is in no mood to give up being Prime Minister. His two main goals – turning round public services and taking Britain into the euro – remain unfulfilled. The paradox is that he is more confident than ever, getting into his stride after five and a half years, and perhaps at the height of his power. The Tory Opposition is still struggling to come to terms with the Blair phenomenon.

The idea that Mr Blair would step down before next general election is laughable to the Blairites. "He is not going to be bounced out of his job," said one close ally. "The pressure is certainly there, but it's senseless. It is more likely to make Tony stay on."

With so much unfinished business on Mr Blair's agenda, a more realistic scenario is that Mr Brown delivers a euro referendum next year in return for a promise that Mr Blair will stand down some time after the next general election. This would delay by four or five years the agreement the Brownites claim was struck at the Granita restaurant in Islington in 1994.

Mr Brown may not like it, but he may have to lump it. Insiders say the relationship, which has been the cornerstone of New Labour, has hit its rockiest patch ever. Although Mr Brown consulted Mr Blair more closely over yesterday's package than some of his previous statements, some Blair aides rather suspiciously believed the Chancellor was happy to spread the load of the bad news.

There is nothing new about differences between the people who hold the keys to Numbers 10 and 11; they come with the tenancy agreement. In the past, Mr Blair and Mr Brown have been better than their predecessors in both main parties at keeping their divisions in the shadows, often denying reports of splits when they were true.

What is significant is that in recent weeks, important differences between the one-time soulmates on at least four separate issues have emerged. The Chancellor has openly challenged the Prime Minister's stance on foundation hospitals, university top-up fees, specialist schools and pensions.

A separate Brown agenda on reforming public services is emerging. Some Blairites see this as manoeuvring ahead of The Big Discussion the two men will have in the New Year on the euro, a pot into which the explosive question of the succession is inevitably going to be thrown.

The biggest threat to Mr Brown's hopes of becoming Prime Minister may be the passage of time. He would naturally have wanted Mr Blair to stand aside in good economic times rather than bad. There are ominous echoes of 1994; Mr Brown was unluckily in the wrong job as a cautious Shadow Chancellor when Mr Smith died while Mr Blair was shining as Shadow Home Secretary. Will history repeat itself unless everything comes right for the British economy next year?

The delay to his ambitions also allows more time for another potential successor to Mr Blair to emerge, perhaps even someone outside the existing candidates of Mr Blunkett, Alan Milburn and Mr Clarke.

And yet Mr Brown remains the front runner, head and shoulders above the chasing pack. The long and winding road may well lead to the door of Number 10; but he will have to be patient for a while yet.