Boris Johnson said it was "unlikely" to be axed in the current climate, but warned that in the long term the City must be able to compete internationally.
Earlier this month David Cameron said he still regarded the rate – introduced by Labour in 2010 – as a "temporary" measure following reports that the Government had accepted it could not be axed before the 2015 general election.
Asked if the 50 per cent rate was "here to stay", he told Sky News' Murnaghan programme: "It certainly looks that way, we have got to be realistic about that. I don't see any sign of it going soon.
"All my job is to do is to argue that in the long term you can't hope for London, a great international market city, to compete with other capitals if we have tax rates significantly higher than our major competitors, and that's a point that I think it's my duty to make.
"I'd like to take people on low incomes out of tax, I'd like reduce tax across the board."
He added: "I think you are probably right in your political analysis Dermot (Murnaghan) that to go for the 50p tax rate in the current climate is probably unlikely."