The last months of Tony Blair's time in office as Prime Minister were always going to be an uncertain time in British politics. Quite how uncertain is now becoming awkwardly apparent. The past few days have highlighted the difficulty. Mr Blair is back in the country, having cut short (by a day) his holiday in Florida to attend to a difficulty that had cropped up in the Northern Ireland talks. He visited a hospital to talk about the undisputed improvements in treating heart disease. Neither on his return, nor during his hospital visit, would he say one word on the single most newsworthy event to take place during his absence: the execution of Saddam Hussein.

This is the one subject that he might have been expected to comment on, given that he has repeatedly cited the downfall of Saddam as the achievement that made the invasion of Iraq -in his view - worthwhile. Yet he and his office batted away criticism, insisting that he would comment in his own sweet time.

At the weekend, it was Gordon Brown, the Chancellor and Mr Blair's presumed successor, who stepped into the breach. Not only did he give the New Year BBC television interview that has been given by the Prime Minister in the past, he commented on the manner of Saddam's execution - describing it as "completely unacceptable" - and he set out priorities for "the next 10 years", which in several respects implied sweeping criticism of the way the country is run at present.

He called for a "new style of government for the future" and he and promised "frankness" in dealing with the United States. In very many respects, this interview - in its staging, its wide range and its serious tone, was a prime ministerial interview - except that Mr Brown does not, yet, occupy that position. He is still Chancellor of the Exchequer. When the Prime Minister's press office rushed out a statement defending Mr Blair's silence on Saddam's execution, it was not only Labour ministers and MPs who could have been forgiven for asking who exactly was making the running.

Mr Blair's shadowy presence since his return from holiday and Mr Brown's high profile interview have fostered the impression that we now have two prime ministers, each with a different idea of how to exercise authority and a different set of priorities. This is something rather different from an orderly transition in which Mr Blair progressively - and graciously - cedes more and more areas of policy to a designated successor. It is the old Blair-Brown rivalry back in evidence again, sharpened by the recognition, as Parliament reconvenes, that the final countdown to Mr Blair's departure has begun.

The present situation is fraught with confusion and tension. Yet Mr Blair seems intent on pursuing his grand plans - for Northern Ireland and his consultation groups - to the apparent exclusion of responses and decisions that are less to his liking. His silence on Saddam suggested a distaste for the reality in Iraq that his decisions have helped to create. And No 10 was already preparing the ground yesterday for an equivocal response to President Bush's expected change of strategy on Iraq. The message was that the British contingent in Basra would remain unaffected by any policy shift in Washington.

The British political system is ill-suited to functioning as a duumvirate, still less one in which the demarcation of authority is not agreed. Someone has to be in charge. The prospect of many more months of in-fighting and policy stalemate is the best argument there could possibly be for a clean break and an open contest for the Labour leadership, sooner rather than later.