S ir Ian Blair is nothing if not political. At a dinner at the Royal Horseguards hotel in April 2005, to mark the start of a counter-terrorism exercise, he ambled up to me – a Tory member of the London Assembly. We were in the middle of the general election campaign and the Metropolitan Police Commissioner, who had been in post for only two months, had just made a well-publicised and surprising intervention endorsing the Government's plan for identity cards. He asked if he would still have a job in a month's time. "I don't know, Commissioner. Do you want me to put in a good word for you with the incoming Conservative Home Secretary?" was my cheeky reply.

I realised then what a politician London's new Police Commissioner was, an impression compounded a few months later when Sir Ian was among those who rang MPs to lobby for government proposals for 90 days' detention for terrorist suspects. Six months later, in the run-up to the May 2006 London borough elections, where the roll-out of the Safer Neighbourhood system of ward-based community policing was rapidly speeded up, virtually any police constable with a pulse was being made up to sergeant, to meet deadlines imposed at the urging of the Mayor of London. Not for nothing did Sir Ian acquire the reputation at City Hall: "Blair by name, Blair by nature".

In this coming week he will be drawing heavily on that political capital if he is to survive the proposed no- confidence vote at the Metropolitan Police Authority (MPA). My feeling – based on first-hand experience, let alone what has been in the public domain – is that he shouldn't, and won't.

I had already begun to worry about the Commissioner's general judgement a week after the 7 July bombings, before the shooting of Jean Charles de Menezes. I and many others were briefed on the details of the attacks. The Commissioner told us with great confidence (a) that the bombers were quite unknown to the police and (b) what the death toll from the bombs would be. Both turned out to be wrong, which isn't a mortal sin in a rapidly changing and dangerous situation, but if you don't know the facts, why make out that you do?

As the pressure on him mounted following the dreadful events of July 2005, the increasingly embattled Commissioner's judgement constantly slipped. In January 2006 he claimed "almost nobody understood" why the murder of two young girls in Soham was such a big story. Two months later, it emerged he had illicitly recorded telephone conversations with several public figures, including the Attorney General. In February this year, Sir Ian was the guest speaker at the annual dinner of the London Mayors' Association and, in front of a black-tie audience that included the Lord Mayors of London and Westminster, he paid tribute to his generally unarmed force, but was heckled with the line "what about the dead Brazilian?".

Even London's local government leaders were questioning his judgement. The suspicion of a number of guests, at this dinner and two nights later at the Community Security Trust dinner at the Grosvenor House hotel, and even at Buckingham Palace, that Sir Ian was somewhat "the worse for wear" led to media reports that senior officers had written to the MPA, concerned about the Commissioner's social drinking.

The publication of the Independent Police Complaints Commission report (the so-called IPCC2) in August this year, into the events after the Stockwell shooting of de Menezes, began to bring Sir Ian's critics out into the open. A Liberal Democrat member of the London Assembly who commented in a robust – but accurate – way at a Police Authority meeting has received a number of bizarre letters from the Commissioner's ever-loyal staff officer, Caroline Murdoch, and even from the deputy clerk of the MPA, David Riddle. At a reception at City Hall I found myself ushered out on to the balcony by the Commissioner and his staff officer to be asked to "clarify" a column I had written about the role of alcohol in public life. Three days later, I received a letter from the Commissioner ostensibly resolving the issue, only to discover his office had decided to leak the letter to the press with the spin that somehow I had "climbed down". I had done no such thing.

At a reception at the French Ambassador's residence to award the Légion d'Honneur to Trevor Phillips, a senior member of the MPA told me that she had lost any faith in the Commissioner and that the chair of the authority, Len Duvall – a respected politician known for his integrity – had had quite enough.

At its last meeting, the MPA had to go into secret session for an hour and a quarter to discuss the acrimonious row between Sir Ian and his increasingly impressive Deputy Commissioner, Paul Stephenson, over whether or not to accept a bonus. This was enough to discomfit even Sir Ian's closest allies.

The Mayor of London's rush to defend Sir Ian is hard to explain. I do not recall Ken Livingstone defending the SAS when they shot dead three IRA terrorists in Gibraltar in 1988. On the other hand, Livingstone knows that if Sir Ian falls, his London administration's great achievement of reintroducing neighbourhood policing and presiding over a fall in crime in the capital (though only in certain categories) will be tarnished.

Sir Ian's tenure has suffered from one "unfortunate" episode after another. Yesterday, contrary to claims that he has "modernised" the Met, came new allegations that the victims of the botched Forest Gate raid in 2006 had been held at gunpoint again and racially abused.

Sir John Stevens was respected and even adored by his officers, but Sir Ian is considered by many in the rank and file to have shown little leadership. A force that believes in its boss is far better at avoiding such scrapes.

All the soundings are that Sir Ian Blair does not enjoy a majority on his own police authority, on the London Assembly, among borough leaders or, crucially, among law-abiding ordinary Londoners. A well-paid post as professor of criminology at an American university will close this unhappy chapter in the history of an otherwise distinguished organisation.

Brian Coleman is deputy chairman of the London Assembly

Further browsing: IPCC report at www.ipcc.gov.uk/stockwell_faqs_for_website_011107.pdf