Boris Johnson tried to get Britain's most senior policeman suspended after lucrative contracts were awarded to a close friend.

An aide for the London mayor demanded to know why Mr Johnson could not remove Sir Ian Blair from his post as Scotland Yard Commissioner while an investigation is under way into how £3million worth of contracts were awarded to his skiing companion Andy Miller.

But Kit Malthouse, London's deputy mayor for policing, was told the mayor did not have the power to suspend Sir Ian and that 'if the nuclear button is pressed', it could put the inquiry in jeopardy.

Kit Malthouse (left) has said that Sir Ian Blair's position gives him too much power

In an email to Catherine Crawford, chief executive of the Metropolitan Police Authority, he wrote: 'Essentially it's the mayor's decision to suspend or not.'

But the MPA's lawyer David Riddle replied: 'If the nuclear button is pressed, expect it to be crawled over for legal flaws. We cannot risk proceedings unwinding because of flaws in our process.'

Mr Malthouse denied last night that he was plotting against Sir Ian, adding: 'These are internal emails asking what might or might not happen in a hypothetical situation – it's nothing more than that.'

A spokesman for Mr Johnson said: 'Boris Johnson hasn't had any meetings or conversations about the future of Sir Ian Blair and awaits the outcome of any investigation.'

The emails sent over the weekend reveal apparent tensions between the new Tory mayor and Sir Ian, dubbed New Labour's favourite policeman.

Though the Commissioner reiterated his healthy working relationship with Mr Johnson yesterday, it is no secret that the Mayor felt he should have resigned last year after his force was convicted of health and safety breaches over the shooting of Jean Charles de Menezes.

It is claimed Boris Johnson wanted to oust the Met commissioner

Sir Ian, who denies any wrongdoing, expressed concerns at his monthly press briefing that the role of Commissioner was becoming increasingly politicised.

He said: 'Most senior officers are concerned that the office of Commissioner is becoming a matter for high politics, which it has been for quite some time.'

Sir Ian added: 'There is concern about the politicisation of the office of the commissioner. I find that inappropriate and I will continue to stand in office because that is my job - to make London safer.'

Yet ironically it was he who lent Tony Blair a police Range Rover emblazoned with 'Vote Labour' stickers at the last election.

He has also supported extended detention without trial for terror suspects, at one point backing 90 days.

The email discussion over Mr Johnson's powers took place as the Metropolitan Police Authority's professional standards committee arranged to have an emergency meeting about the Andy Miller issue.

After discussing it on Monday, the committee launched an inquiry and asked Jacqui Smith to appoint an external investigator to head up the panel.

The appointment is expected to be announced shortly. It is understood that the Home Secretary will ask Sir Ronnie Flanagan, Chief Inspector of Constabulary, to do the job.

It remains possible that the external investigator, along with members of the professional standards committee, will choose to suspend Sir Ian during their investigation.

Kit Malthouse, London's deputy mayor for policing, appeared to express frustration that the mayor, as the only publicly elected representative, was not able to remove him from his post himself.

He wrote in an email to the MPA's chief executive Catherine Crawford: 'There is too much decision making by committee on stuff like this and it gives individual members far too much sense of power.

'Essentially it's the Mayor's decision to suspend or not.'

But the MPA's lawyer David Riddle, describing the situation as 'uniquely serious', replied in an email: 'The political imperatives are understood but if there is to be action against the Commissioner it had got be lawful - that means following due process by the MPA under the regulations.

'If the nuclear button is pressed, expect it to be crawled over for legal flaws. We cannot risk proceedings unwinding because of flaws in our process.'

Mr Malthouse yesterday denied he was plotting against Sir Ian and said that he was just investigating the 'accountability and methodology' of the procedure.

He said: 'These are internal emails asking what might or might not happen in a hypothetical situation - it's nothing more than that.'

He said the issue of suspension was raised as a 'possibility of what might happen in the future' and added: 'That doesn't mean that it was actively under investigation.'

A spokesman for Mr Johnson said: 'Boris Johnson hasn't had any meetings or conversations about the future of Sir Ian Blair and awaits the outcome of any investigation. He continues to work with Sir Ian in fighting knife crime in London.'