Boris Johnson says coalition 'doomed to succeed' and last to 2015
Boris Johnson has said the coalition government will last to 2015, telling the BBC it was "doomed to succeed".
The Mayor of London said there was "no reason" for the Conservative and Liberal Democrats to go their separate ways before then.
Some Conservative MPs have been speculating that the arrangement will break up well before the scheduled date of the next election in May 2015.
This follows tensions over reforms to the House of Lords and other issues.
Prime Minister David Cameron and his deputy Nick Clegg have made a joint announcement on rail investment on Monday in a show of unity following a week of wrangling over the House of the Lords.
'Moment of separation'
The largest rebellion in the coalition' s history over plans for a mainly elected Lords - in which more than 90 Conservative MPs voted against their leadership - has prompted fresh speculation about the government's longevity.
Mr Cameron has acknowledged "profound areas of disagreement" between the two parties, but told the Sunday Times they must work together in the national interest over the next few years.
Asked about the coalition' s future, Mr Johnson told BBC Breakfast that "logically it must be true that at some stage by 2015 there will have to be a decision to part company and to campaign on a different prospectus".
He added: "But I don't see any need particularly for it to happen urgently.
"It is a marriage which is doomed to succeed. It will continue absolutely until the last moment when it is necessary to part in order for two parties to go into two campaigns on separate manifestos."
On Sunday a senior Conservative backbencher said it was "very likely" the coalition would end before the start of the next general election campaign.
"I think it would be logical and sensible for both parties to be able to present their separate vision to the public in time for the public to form a clear view before the election," Graham Brady, chair of the influential 1922 backbench committee, told BBC Radio 4's The Westminster Hour.
"Of course, it is always possible that that moment of separation could come sooner. It's very difficult to predict when that might be."
Senior Tories, including the former defence secretary Liam Fox, have accused the Lib Dems of sidetracking the government over Lords reform and have urged Mr Cameron to assert his authority on key issues like the economy, Europe and welfare.
But former Lib Dem leader Sir Menzies Campbell has said the coalition was a "marriage of inconvenience".
He has suggested many Lib Dem MPs would find it "hard to swallow" proposed changes to Commons constituency boundaries in 2015 - thought to favour Conservative chances of getting a future majority - if changes to the House of Lords do not proceed.
The BBC News Channel's chief political correspondent Norman Smith said both Mr Cameron and Mr Clegg were fully committed to the coalition and neither wanted an early election.
The leaders' immediate task, he added, was to "calm their parties" down after recent events and demonstrate their focus on the economy and getting the UK out of recession as quickly as possible.