Mr Johnson said Sir Ian had "too many distractions"
London Mayor Boris Johnson has denied a "dangerous precedent" has been set by the resignation of Metropolitan Police Commissioner Sir Ian Blair.
Mr Johnson told BBC One's The Politics Show there were "too many distractions" preventing Sir Ian from doing his job.
Sir Ian cited a lack of support from the mayor as the main reason behind his decision to step down on 1 December.
The Tory mayor said claims political interference lay beyond the departure were "balderdash, piffle and tripe".
Home Secretary Jacqui Smith has accused the mayor of acting without authority or respect by pushing Sir Ian out for political reasons.
The normal procedure for removing the commissioner involves the Metropolitan Police Authority (MPA) - now chaired by the mayor - seeking the home secretary's approval.
But Mr Johnson also said he did not believe a "constitutional precedent has been set" by Sir Ian's resignation on Friday.
He said he had had a discussion with Sir Ian about how to improve "operational effectiveness" and said it seemed like a good time to give someone else the opportunity to lead the force.
"There were too many distractions and I think Sir Ian accepts that as much as anybody," he said.
Mr Johnson added he had agreed with Sir Ian not to go into an "item by item analysis" in public.
The Home Office and the MPA will now draw up a shortlist for Sir Ian's successor.
Met deputy commissioner Sir Paul Stephenson will take over as acting head should no candidate be appointed by the time Sir Ian leaves the post.
Tony Travers, a local government expert from the London School of Economics, told the Politics Show the events surrounding the resignation appeared to create a "constitutional mess".
He said: "The implication is that if the home secretary appoints the commissioner she would get rid of him."
Describing Mr Johnson's involvement he said: "It's a big leap in the dark as a political decision but it's a demonstration of raw power and its ripples will be felt for a long time to come."
Conservative party leader David Cameron, however, said it was time for Sir Ian to go.
He said: "Senior police officers like other public servants have to be publicly accountable - if something goes wrong and mistakes are made you have to carry the can.
"It was time for a change."