The London Mayor said he withdrew support from Metropolitan Police Commissioner Sir Ian Blair to improve policing in the capital.
He insisted no constitutional precedent was set by apparently undermining the prerogative of the Home Secretary to appoint the head of Scotland Yard.
Sir Ian resigned yesterday blaming Mr Johnson and said he was not leaving because of any failings.
Home Secretary Jacqui Smith accused the Mayor of acting without authority or respect by pushing Sir Ian out for political reasons.
But speaking at City Hall today, Mr Johnson said he made the decision in the interests of Londoners after "widespread consultations".
He said: "I think some of the analysis I have read this morning has been absolutely outlandish.
"There is not and has not been any kind of party political plot by the agents of the Tories, Labour and Lib Dems. There is no party political element to this.
"I simply thought after long reflection and widespread consultation that it was a good opportunity for someone else to offer new leadership, stability and increased operational effectiveness, let me put it like that, increased operational effectiveness for the Metropolitan Police and that was not an opportunity that I could let go by."
The procedure for removing the Met Commissioner is that the Metropolitan Police Authority (MPA), its board of governors, must seek the home secretary's approval.
But Sir Ian's sudden resignation at the hands of the Mayor has led to claims the role is more politically vulnerable than ever.
Some claimed it is now almost inevitable that there will be a change of Commissioner with every new London administration.
All eyes will now turn to City Hall on Monday when Mr Johnson will chair his first meeting of the MPA.
Former home secretary David Blunkett said: "We need to know when the new Commissioner is appointed that if there was a change of Government there wouldn't be a change of Commissioner on the back of that, purely on the back of that, and we need to know that procedures in future will be followed."
Ken Jones, president of the Association of Chief Police Officers (Acpo) and potential successor to Sir Ian, was also concerned.
He said: "We are concerned, particularly at the senior level. My view is chiefs and commissioners are not in a dissimilar position to judges.
"They need to balance constantly vested interests, populism, various pressures and ultimately they are accountable to law.
"We need politicians of all persuasions to recognise the dilemmas they face and to give them their support."
Speaking on BBC's Question Time last night, Ms Smith said: "There's a process in place that the mayor chose not to respect.
"What is important when you are both choosing and when you're supporting somebody that you're asking to do a job like that, is that you keep party politics out of it.
"You need to work alongside people and, frankly, you should put some time and effort into that.
"The mayor said on the first day in his job he didn't feel he had confidence in Sir Ian and that's why he took the decision to resign."
Brian Paddick, a former Met deputy assistant commissioner, said Ms Smith was instrumental in Sir Ian's departure.
He told the BBC: "On the day the Mayor becomes the chair of the Metropolitan Police Authority he says boo and the Commissioner jumps.
"Not only that, it is actually only the Home Secretary that could force the Commissioner to leave and therefore the Home Secretary could have turned round and said to Ian Blair and to the Mayor 'I'm sorry, you don't have the power, Mayor, to do that. I want the Commissioner to stay'. But she didn't, she allowed the Commissioner to go."
Announcing his resignation, Sir Ian said: "I am resigning not because of any failures of my service and not because the pressures of the office and the many stories that surround it are too much.
"I am resigning in the best interests of the people of London and of the Metropolitan Police Service.
"However, at a meeting on Wednesday the new Mayor made clear, in a very pleasant and determined way, that he wished there to be a change of leadership at the Met."
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