Boris Johnson is set to vote against changing Britain's electoral system in a referendum planned for next May.
A City Hall source said the Mayor had "mellowed" in his opposition to the alternative vote — in which voters list their candidates by preference — but will still support keeping the first-past-the-post system for Westminster elections.
However, Mr Johnson will not lead a campaign to defend it in the run-up to the referendum.
The source said: "He will vote against AV. He still thinks first-past-the-post, historically has produced strong governments."
Mr Johnson believes he would have been elected Mayor more comfortably if first-past-the-post had been used rather than AV as more of Lib-Dem candidate Brian Paddick's second choice votes went to Labour's Ken Livingstone in 2008.
If the Mayor were to campaign to keep first-past-the-post, it could spark fresh speculation that he was positioning himself as a rival to David Cameron. Even though the Prime Minister opposes AV, Tory MPs believe that he will not take a high profile role opposing it.
In talks with Mr Cameron yesterday the Mayor argued that the capital should be treated more leniently in spending cuts as he had been making efficiency savings for two years.
