Leading article: The travails of a Prime Minister whose authority is ebbing away

Mr Blair's ambition is to leave on a high note. Every day he remains in office, his power and standing are diminished

This has been a truly wretched week for Tony Blair. Not surprisingly, the catastrophe in Iraq has been the source of many of his problems. It was a gross misjudgement by the Prime Minister to stay away from the House of Commons debate on the subject. His decision to attend a minor CBI conference instead screamed distorted priorities.

Yet Mr Blair's troubles in this area go considerably deeper. It became clear this week that the White House's new policy on Iraq bears little similarity to our own. Some of Mr Blair's cabinet ministers - notably Peter Hain and Hilary Benn - have already broken ranks. As Iraq slips further into chaos, more will surely follow.

The situation is little better for the Prime Minister on the domestic front. Even an issue as apparently innocuous as the future structure of the Home Office is causing problems. Asked repeatedly by David Cameron at Prime Minister's Questions this week whether he approves of a plan floated by the Home Secretary to divide it into two ministries, Mr Blair refused to commit himself. This has been interpreted as a sign that, with Mr Blair's time in office fast running down, it will not be his decision to make.

The crisis in our prisons is damaging the Prime Minister too. As Home Secretary, John Reid has borne the brunt of the criticism. But Mr Reid has been in his post less than a year. Mr Blair has had a leading role in formulating policy on criminal justice since 1997. The present overcrowding did not spring up overnight. It is the result of almost a decade of ill-considered and reactionary policies for which Mr Blair must take primary responsibility.

Worst of all perhaps, Mr Blair's political authority is slipping alarmingly. In the dispute over Catholic adoption agencies Mr Blair was keen to accommodate the Catholic Church. But he did not succeed. Even former loyalists such as Alan Johnson and Lord Falconer were prepared to defy him.

The contest for the deputy leadership has created a new political dynamic. The likes of Alan Johnson, Hilary Benn and Peter Hain are more concerned with winning the support of the Labour Party rank and file than supporting their leader. They know that Mr Blair's political patronage is nearly exhausted.

Humiliatingly, the Prime Minister's personal probity is under question too. The Metropolitan Police's "cash for peerages" investigation shows no sign of fizzling out. Last Friday, Ruth Turner, Mr Blair's "gatekeeper", was arrested and there are fresh allegations of a "hidden" computer network at Downing Street from which emails have been deleted. This shadow extends not just over Mr Blair, but over the entire government. It will continue to do so while he remains Prime Minister.

In this bleak context it is not surprising that some ministers are advising the Prime Minister to announce now that he will leave office after the Scottish elections in May. Yet announcing a timetable would hardly help Mr Blair personally. It would merely accelerate the decline in his authority.

Mr Blair's ambition is to leave on a high note. He is said to be hoping for a deal on Northern Ireland power sharing, evidence of progress in Iraq or perhaps a US commitment to a new international treaty on climate change in the next few months. All are looking unlikely. The reality is that time is not on the Prime Minister's side. Every day he remains in office, his power and standing are diminished. In the rancorous twilight of his premiership, the one sensible move left open to Mr Blair would be to bow out sooner than people expect.