YATES of the Yard has a lot to answer for.

The investigation by Assistant Commissioner John Yates into the alleged cash for honours affair has dragged Tony Blair so far into the mire that he has little chance of climbing out again. Whatever integrity he had left is now mud-stained and torn.

Hardly anyone has any time for Tony Blair nowadays, and the babble the Prime Minister hears from dawn to dusk is the chorus of complaint, the invitations to leave now rather than later. They're all at it, his own people, the other side, the columnists and the commentators, all aiming poisoned arrows at a sitting target.

Much of the blame for this state of affairs can be laid at the Prime Minister's own door (which still features a number ten, although not for much longer). By announcing that he would not seek a fourth term as Prime Minister, he shot holes in his own authority. He also sacrificed himself to the dark agitations of his own party, and the chained ambitions of Gordon Brown.

Tony Blair has stretched the nation's patience, and his legacy is now deeply tainted by suggested sleaze, the bloody Iraq aftermath, and memories of spin-loaded politics, but there is a case to be made on his behalf on this inquiry.

Never mind, for the moment, his achievements or otherwise, but think instead of what is going on.

The Prime Minister is being further smeared by an interminable police investigation of questionable benefit. Mr Yates has a job to do, but he is taking his time about it. Many times now, the assistant commissioner has hinted darkly at a conclusion, but instead the investigation has ground on, without result. There have been arrests and releases, and Mr Blair himself has spoken to the police on two occasions.

Part of the problem lies in the nature of what is being investigated. Has a crime been committed? If this is a whodunit, then we have to wonder "who done what to whom".

The inquiry is based on a dormant 1925 act which prohibits the sale of titles. Yet what is being investigated is, in effect, a fact of political life at Westminster. A stained and grubby fact, perhaps, but still an uncomfortable truth. Labour is not alone in handing out favours to generous friends by elevating to the Lords those who have "helped", and the Tories have nothing to boast about on this matter.

In that sense, the Yates investigation seems out of all proportion - and the way this inquiry has been run appears to be harming our democratic institutions.

Like anyone else, Tony Blair deserves to be presumed innocent until proved otherwise. Instead, the longer this inquiry takes, the guiltier he seems. He has no chance to put his side or, if you prefer, to spin things his way. He just has to keep quiet and let the assumptions of guilt build up.

Maybe Tony Blair is guilty of something, and maybe all this will bring down New Labour, denying Gordon his long-cherished turn at the wheel. But it is still a "maybe".

This is not, just to make clear, a defence of Tony Blair so much as a criticism of the police investigation. How fine that we live in a country so untroubled by crime that the police are free to spend so much time on a questionable investigation. Isn't there any real crime they could get to work on?

It is hard, too, not to escape the conclusion that Mr Yates and his team must be secretly loving all the headlines and the media attention. Arresting proper criminals is nowhere near as much fun. Ten months and counting, and nothing to report yet.

Tony Blair should be allowed to get on with his job for now - and then leave with whatever good grace he can muster, to contemplate the spreading stain of his legacy.