The Foreign Office may now be the best Mr Brown can hope for

When the Chancellor of the Exchequer went to Africa in his chinos last month, he was not the first Gordon to come a cropper. He avoided the fate of General Gordon, whose head was paraded on a pike after the fall of Khartoum to the Mahdi rebellion in 1885. But it may have been the beginning of the end of his ambition to succeed Tony Blair as Prime Minister.

When the Chancellor of the Exchequer went to Africa in his chinos last month, he was not the first Gordon to come a cropper. He avoided the fate of General Gordon, whose head was paraded on a pike after the fall of Khartoum to the Mahdi rebellion in 1885. But it may have been the beginning of the end of his ambition to succeed Tony Blair as Prime Minister.

This was not the conventional view at the time. What was striking was how prime ministerial he looked, how comfortable on the world stage and how engaged with the human dimension of poverty on the poorest continent. But if Brown eventually joins the long and distinguished list of claimants to be the best prime minister the Labour Party never had, we may look back to this period as the time when his tide began to recede. This is not primarily because the Americans rejected his plans for a new form of aid funding at yesterday's London meeting of finance ministers from the rich countries. The cynic might regard a bust-up with the Bush administration as helpful to his designs on the leadership, but that is surely too cynical. Progress on African development is one of the British Government's two priorities for this year, in which it holds the chair at meetings of the G8 club of rich nations. And if the Americans don't play, then there is no progress. What does seem cynical is the competition between Blair and Brown to be seen as the prime mover of Africa policy. Blair's authentic demand, that "I, personally, should be associated with it", from a private memo on crime policy in 2000, rings through the years, as he and Brown try to be photographed with Nelson Mandela, Bob Geldof, Bono and crowds of African children. And if Brown's plan stays smouldering on the ground, we can be sure that Blair will be as far away from the wreckage as possible.

But that is politics. There is something to be said for the argument put to me by one cabinet minister, who described the Blair-Brown relationship as one of "creative" tension and said that they were at least trying to outdo each other in pushing towards a worthy objective.

The danger to Brown is that, by going to Africa and by trying to compete for the limelight of global compassion, he has made it easier for Blair to move him after the election. Until last month, I had thought that it would be awkward for the Prime Minister to offer Brown the Foreign Office, because he was so popular and well regarded where he was. Now that can be offset by Blair's saying how much he wants his "brilliant" Chancellor to transfer the skills that have started us on the road to ending child poverty to a global scale.

Maybe that is what Brown wants. It is certainly possible to make that case. Hence the mystery over last Thursday's articles in two newspapers owned by Rupert Murdoch. Trevor Kavanagh, the political editor of The Sun, reported that that Brown was ready to become Foreign Secretary after the election. Meanwhile, in The Times, Peter Riddell set out rather persuasively why he should want to. Blair's people and Brown's people accuse each other of inspiring simultaneous articles that would otherwise be a bit of a coincidence. The Prime Minister's spokesman even broke with precedent to say that Blair had had no meeting or discussion with Murdoch, who had been rumoured to be the common source.

The genesis of the two articles does not matter so much as the fact that they reflect something that is really going on. It must now be more likely than not that Blair will move Brown if Labour wins the election. There are two big reasons. The Prime Minister is focused on the general election campaign, but an important part of his mind is looking ahead to how to maintain momentum afterwards if it is successful. Since the beginning of December, when the French Socialist Party decided to support a Yes vote in this summer's referendum, he has been increasingly alarmed by the approaching train crash of the European Constitution.

His initial calculation, that the constitutional treaty was most unlikely to be ratified, still holds good. A helpful aide-memoire put out by the Centre for European Reform last week lists three referendums to be held this year - in France, Ireland and Denmark - in which the odds of a Yes vote are put at only 55 per cent. Then there is Poland this autumn and the Czech Republic next year, both put at 50-50, before we get to the UK, at 40-60 against. But Blair has to start taking seriously the possibility that all the other member states will say Yes. And, in doing so, he is beginning to realise that even a French or Polish No will only present new difficulties for him. Either way, he cannot allow his rival for the leadership to sit quietly in the Treasury waiting for it all to go horribly wrong. Brown must therefore, as Foreign Secretary, take responsibility for fighting the referendum campaign himself - which, in turn, will make a British Yes vote more likely.

The second reason why Blair is likely to move Brown is because, I think, he would prefer a new Chancellor. A curious feature of Kavanagh's and Riddell's articles was that they did not mention who might take over the second most important job in the Government. There are two main candidates: Alan Milburn and John Reid. The question of what job Milburn might get after the election has been one of the hidden puzzles of politics since his return to the Cabinet in September. He may not know himself what he wants. But we can deduce what Blair wants, and either Milburn or Reid could provide it. The Prime Minister has come to regard public service reform as the big mark on history that he can make in Labour's third term. His hopes of taking Britain into the euro have vanished, while his record on foreign affairs will probably always be disputed because of the Iraq war. That leaves the transformation of public services by bringing in market-based reforms. And Brown has become a serious obstacle to this objective. Robert Peston's biography of the Chancellor - with which his friends co-operated on the obvious assumption that he would be Prime Minister by the time of its publication - sets out his very different views of how the public sector should work.

It may be that Brown has seen Blair coming and is preparing for a move to the Foreign Office so that it looks less like a demotion, or to head off a worse offer. It would at least open him out as a politician and - to be cynical - it might get him out before the public finances become sticky. But the Foreign Office is not a good platform for leadership ambitions. Brown's engagement with the problems of Africa is welcome, but it may also be his swansong.