The Mayor of London’s decision to force out Sir Ian Blair is the most significant display of political power by a Conservative executive since John Major sacked Norman Lamont in 1993.

Not only does it mark the end of the line for London’s top policeman, but also the beginning of a more aggressive phase of the Johnson mayoralty, which will actively highlight failings in the Labour Government.

Aides said that Boris Johnson had concluded that the growing number of newspaper allegations and the inquest into the death of Jean Charles de Menezes was distracting the Commissioner of the Metropolitan Police from his job.

Mr Johnson’s office insisted last night that the decision had been taken only in recent days, emphasising that the pair had enjoyed a good working relationship, travelling round London together in squad cars. When Sir Ian arrived for a routine meeting with the mayor on Wednesday the police chief had had “no clue” what was about to happen, the office said.

But the real significance of the decision lies in the mayor now having the confidence to use his powers to highlight what he sees as the Government’s shortcomings.

In the early hours of May 3, the incoming mayor declared in his victory speech at City Hall that he wanted to “put aside party difference” to try to make Greater London “greater still”.

But now Mr Johnson’s aides say they hope that his decisions will draw attention to the “crossover between a tired and drifting Government unable to take controversial decisions and a mayoralty with a huge mandate and clear direction”.

“The biggest charge against Gordon Brown is that he is a ditherer,” a Johnson aide said. “All the allegations against Blair stack up and no one in government can make a decision.”

The Mayor of London is now heading for a showdown with Jacqui Smith, the Home Secretary, who made clear that she was prepared to overrule his choice of replacement for Sir Ian Blair. Last night Ms Smith attacked Mr Johnson on the BBC One programme Question Time over his handling of Sir Ian’s departure. She said: “There’s a process in place that the mayor chose not to respect.”

Mr Johnson is now heading for a trial of strength over the appointment. It will be the first time that a Conservative mayor has had to recommend a candidate to a Labour Home Secretary.

Under the rules, the Metropolitan Police Authority, which is chaired by the mayor, will make a recommendation to Ms Smith. Under legislation passed in 1996, the Home Secretary makes the final decision.

But the chances of the issue becoming mired in party politics rose last night after both sides claimed their right to have the final say.

Sources close to Ms Smith said that they wanted to reach a consensual agreement but added: “At the end of the day, it will be her recommendation.” The Home Secretary is angry at the way the decision was made and made it clear that she thought that Sir Ian had played a “superb role” and that he had been the victim of “party political football”.

Sir Ian had no idea he would be asked to stand down when he arrived for the routine meeting on Wednesday, which had been scheduled to discuss the agenda for Monday’s meeting of the police authority.

But the mayor’s office hit back, warning the Government: “It could clearly be obscenely inappropriate to pick someone who is not 100 per cent supported by the mayor and the chair of the Metropolitan Police Authority.”

Two of Sir Ian’s biggest backers attacked the decision. Ken Livingstone, whom Mr Johnson defeated in the mayoral election in May, said: “Boris made an election issue out of saying he had no confidence in Sir Ian. I suspect we will look back to this day as the day when the role of commissioner became party-political.”

David Blunkett, who as Home Secretary, appointed Sir Ian in 2004, said: “This is rule by autocracy and a dangerous precedent, politicising the police force in Britain.”

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