Prime Minister David Cameron was quick to welcome the announcement yesterday, saying he had always wanted to see “my star players on the pitch” during the election campaign.
However, Mr Johnson’s declaration immediately raised the prospect that he could emerge as a rival for the Conservative leadership if the party is defeated – or again fails to secure an overall Commons majority.
Ladbrokes responded by slashing the odds of Mr Johnson becoming the next Conservative leader from 5/1 to 9/4, while Labour said it was a clear sign the Tories were turning on themselves with Mr Cameron powerless to intervene.
The mayor provocatively used a question-and-answer session at the end of a speech in which he also highlighted the “attractive” prospects for Britain outside the European Union if Mr Cameron fails to secure promised reforms.
“I think we’ve danced around it an awfully long time now and, as you know, the Prime Minister ages ago said he would welcome me back and has also been pretty clear that I can’t endlessly go on dodging these questions as I’ve tried to do,” he said.
“So, let me put it this way – I have not got any particular seat lined up but I do think in all probability – since you can’t do these things furtively, I might as well be absolutely clear – in all probability I will try to find somewhere to stand in 2015.”
While he added that it was “highly likely” that he would be unsuccessful in his search for a seat, few at Westminster doubt that his announcement will see him return to the centre stage of politics.
Mr Cameron, currently on holiday, wrote on Twitter: “Great news that Boris plans to stand at next year’s general election – I’ve always said I want my star players on the pitch.”
Nevertheless, the news will be greeted with mixed feelings in Downing Street. While Mr Johnson’s undoubted popular appeal will be an electoral asset, his return to Westminster will galvanise Eurosceptic Tory MPs.
For Labour, Sadiq Khan, the shadow minister for London, said: “Boris Johnson’s announcement reveals how weak David Cameron is and how out of touch the Tories remain.
“Rather than focusing on helping the millions of Britons suffering from the cost-of-living crisis, the Tories are increasingly turning inwards, focused on leadership battles to come.”
Mr Johnson refused to say whether he had discussed his plans with Mr Cameron, although he said it was largely at the Prime Minister’s instigation that he decided to “have a crack”.
“My conversations with the Prime Minister are many and various and I don’t propose to go into them,” he said.
Mr Johnson also refused to be drawn on where he might stand, although there is intense speculation he could go for Uxbridge where former deputy chief whip John Randall is standing down.
Culture Secretary Sajid Javid said Mr Johnson’s announcement was “fantastic news”.
“He’s a huge asset for London, he’s been a huge asset for the Conservative Party and if he does get elected in 2015, he’ll be a huge asset for the country.
“I know the Prime Minister will be absolutely delighted with this news.”
Mr Johnson said that he wanted to continue supporting Mr Cameron, whom he described as a “brilliant” Prime Minister.
He made clear, however, that he would relish the prospect of ministerial office once he has completed his term as mayor which runs to 2016.
“I would like to see if I could be useful again in Westminster, because when I was there last time, after all, we were in opposition and I spent quite a lot of the time bashing away at Tony Blair and Gordon Brown and pretty miserable it was,” he said.
“I’d love to see what it’s like in government.”
Conservative former international development secretary Andrew Mitchell said: “He reaches parts of the voting public which others do not reach.”