Leading article: A commissioner who tried to be politician and policeman

Sir Ian Blair's time ran out when the Conservatives took power in London

It was probably inevitable that Sir Ian Blair would have to resign as Britain's most senior police officer once the new Conservative Mayor of London, Boris Johnson, took control of the Metropolitan Police Authority. Mr Johnson has been, at best, lukewarm in his support for the beleaguered Commissioner of the Metropolitan Police who has, in recent months, been beset by a series of controversies, some of them serious and some exaggerated by enemies in the Conservative Party and sections of the press.

The case against Sir Ian was a telling one. His three and a half years as Britain's top policeman have been blighted by controversy, which began when his officers shot dead Jean Charles de Menezes after mistakenly identifying him as a suicide bomber. It was a difficult situation and Sir Ian made a number of serious mistakes. But he was cleared of the specific accusation that he obstructed efforts by the Independent Police Complaints Commission to investigate the tragedy. And, although a jury convicted the Metropolitan Police of violating health and safety laws, it accepted that this was an "isolated breach under quite extraordinary circumstances".

More clear-cut was the Commissioner's failure to defuse a bitter war with senior Asian officers in his force who accused him of handing out senior jobs to a "golden circle" of white officers less well qualified for promotion than black and Asian candidates. Whether or not there was any substance to the claims, they were clearly not handled well. The in-fighting in the upper ranks at Scotland Yard was bad for overall morale. There were also questions over his awarding a PR contract to a firm run by a long-time friend.

That said, Sir Ian Blair handled the worst terrorist outrage London has ever undergone with a cool confidence that calmed the country's nerves. His command of the nation's counter-terrorism strategy has been authoritative. Aided by able senior staff, he made massive strides in purging the Met of the "institutional racism" described by the 1999 Macpherson report, working hard to boost the number of Asian, black and female officers. Indeed there is some evidence that the mutinous disloyalty of other senior colleagues stemmed from opposition to reforms of which Sir Ian can be justly proud.

One of the ambiguities of his record is that, while the statistics show that he leaves London a safer placer than he found it, many Londoners remain sceptical. His redistribution of resources to neighbourhood policing has cut robberies by 21 per cent, but a spate of high-profile teenage knife murders belies an official 10 per cent fall in violent crime. The effectiveness of the controversial community support officers has yet to be proved; suspicions persist that it is policing on the cheap.

If there were errors of judgement – such as his insensitive comments about the Soham murders or his secret recording of a phone conversation with the Attorney General – the biggest of these was probably his closeness to New Labour, even if he did withdraw his support for 90-day detention without trial. His identification with the government of the day brought into being a coalition of the Conservative opposition and a media coterie that set out relentlessly to undermine him. Once the Conservatives took power in London, his days were numbered.

There is a lesson in that. What London needs to replace him is a policeman not a politician, especially one whose political skills left something to be desired. The politicians who choose Sir Ian's replacement must not make the same mistake again.