Sir Ian Blair had been Commissioner of the Metropolitan Police for five months when Jean Charles de Menezes was shot dead by police marksmen in South London. He has faced demands for his resignation ever since.

His responses are by now well practised. He believes that near-constant pressure to quit is an occupational hazard to be shrugged off if not actually ignored. And he believes mutinous disloyalty from senior colleagues is an inevitable result of radical reforms of which he is fiercely proud.

The trouble for Sir Ian is that his reforms have not made him indispensable. Nor can he be sanguine any longer about the calls for him to go. His support from the Association of Chief Police Officers and the Home Office has crumbled: his contract will not be renewed in 2010. This makes him a lame duck not only in the view of his many critics, but in fact. If his record were spectacular, this newspaper would back his bid to stay in office until the 2012 Olympics and beyond. Unfortunately, it is not.

The commissioner's in-tray is full and daunting. This month, at the inquest into the death of Mr de Menezes, he will face charges of systemic failure by his force on the day of the Stockwell Tube shooting; of allowing Met officers and lawyers to claim afterwards that Mr de Menezes had been acting suspiciously when he had not; and of obstructing efforts by the Independent Police Complaints Commission to investigate the tragedy. He is also the subject of a separate inquiry into claims that he influenced the award of contracts worth £3 million to an old skiing friend, and his most senior Asian officer, Assistant Commissioner Tarique Ghaffur, has accused him personally of racial discrimination.

The Ghaffur case has prompted gleeful suggestions that Sir Ian is the victim of his own political correctness. This is unfair. Great strides have been made on his watch towards purging the Met of the “institutional racism” described by the 1999 Macpherson report. Recruitment of non-white officers has increased fourfold in the past five years, and he deserves much of the credit.

His second achievement, he would claim, is to have driven down crime with a wholesale redistribution of resources to neighbourhood policing. Official figures support the claim - showing a 10 per cent fall in violent crime and 21 per cent fewer robberies since 2005 - but perceptions do not. Recorded crime statistics may be “true” on their own terms, but a public appalled by the plague of fatal teenage knife crime no longer finds them credible. Sir Ian's standing with the seven million Londoners whom he has pledged to protect has suffered gravely as a result.

His record is marred by three other serious failures. First, a misplaced confidence in his own talent for public relations has backfired with a series of well-publicised gaffes and an excessive concern with his treatment by an increasingly mistrustful media. Secondly, his style and agenda have cost him the confidence of too many of his senior commanders. From Mr Ghaffur's public insurrection to damaging background briefings by pretenders to his job, the symptoms of his loss of authority are manifest.

Thirdly, and most importantly, Sir Ian has politicised his role to the point that neither his force nor the Home Office knows what to make of him. An avowed ally of new Labour, he appeared for two years to be in lockstep with the Government's approach to crime and terrorism - then broke ranks by withdrawing his support for 90-day detention without trial.

London needs a policeman, not a politician, in charge of the Met. Sir Ian has said that he would stand down if he thought staying on would damage the force. The signs of damage are too blatant to ignore.