Boris Johnson has not ruled out becoming a minister in the first year of a Tory government but said David Cameron would not be in favour of such a move.
Mr Johnson insisted he could not see how a ministerial position would be compatible with his job as Mayor of London, which will end in May 2016.
After that the road would be clear for Mr Johnson to be promoted to government if the Tories form it and he becomes MP for Uxbridge and South Ruislip.
But Mr Johnson stressed that his current mayoral position was like running a big budget government department already.
On a visit to the site of a new Intercontinental hotel in North Greenwich, Mr Johnson was asked if there are rules against being a minister, MP and mayor at the same time.
He replied: "I don't know what the rules say, in fact I don't think there are any rules, but the job of Mayor of London is effectively like running a big department of state.
"There's a £16 billion budget or whatever, there's lots of executive stuff.
"What you can do is be an MP, just as you can be an MP and run a big department but I think to do another big budget-wielding department would be pretty tricky."
Asked to rule it out, he said: "I just don't see how.
"It's not for me to rule out, the Prime Minister has already made his views pretty clear about that."
Asked if he had a dream job in the government, Mr Johnson replied: "I've got a dream job it's called Mayor of London.
"As I've explained I have to serve that.
"Obviously I would be very honoured but I've got to do the job of Mayor of London until May 2016."
Mr Johnson also joked that the colour of the so-called Boris bikes for hire in London changing from blue to red was not indicative of what is happening in the General Election campaign.
The mayor said he was still hoping for a Conservative majority on May 7 as voters will focus their minds more.
Asked if he was worried about opinion polls remaining stubbornly deadlocked and whether the hire cycles were an omen, Mr Johnson said: "It's nothing to do with the colour of the bicycles."
"I think really that loads of people have yet to make up their minds, genuinely I'm finding this everywhere I go in the campaign and I think that they will focus, focus, focus I think in the last week or so on that choice.
"Do you want stability, certainty, security, taking the economy forward or do you want to risk chucking it all away?