The former London mayor’s decision not to join the battle leaves Home Secretary Theresa May as hot favourite to be the next Prime Minister.
It came after the shock announcement by fellow Brexit campaigner Michael Gove - who had widely been expected to be Mr Johnson’s running mate - that he was putting himself forward for the leadership.
In a dramatic press conference just moments before the deadline for nominations passed, Mr Johnson said that the next Tory leader would have to unify his party and ensure that Britain stood tall in the world.
And he said: ‘Having consulted colleagues and in view of the circumstances in Parliament, I have concluded that person cannot be me.’
The nominations for the party leadership are Stephen Crabb, Andrea Leadsom, Liam Foc, Theresa May and Michael Gove.
Earlier, Mr Gove - the Justice Secretary who campaigned alongside Mr Johnson for Leave in the EU referendum - said the former London mayor “cannot provide the leadership or build the team for the task ahead”.
The bombshell announcement is a devastating blow for Mr Johnson, who had been expected to announce that Mr Gove would be his campaign co-ordinator.
It leaves Home Secretary Theresa May the clear favourite to succeed David Cameron in the race for No 10.
In a statement released just hours before Mr Johnson was due to formally launch his leadership bid, Mr Gove said he had “reluctantly” concluded that he could not support the ex-London mayor.
‘I have repeatedly said that I do not want to be prime minister. That has always been my view. But events since last Thursday have weighed heavily with me,’ he said.
‘I respect and admire all the candidates running for the leadership. In particular, I wanted to help build a team behind Boris Johnson so that a politician who argued for leaving the European Union could lead us to a better future.
‘But I have come, reluctantly, to the conclusion that Boris cannot provide the leadership or build the team for the task ahead. I have, therefore, decided to put my name forward for the leadership.
‘I want there to be an open and positive debate about the path the country will now take. Whatever the verdict of that debate I will respect it. In the next few days I will lay out my plan for the United Kingdom which I hope can provide unity and change.’
Signs of the tensions that had developed between the two men were exposed when an email from Mr Gove’s wife was accidentally sent to the wrong person.
In it, Daily Mail columnist Sarah Vine warned her husband that he must secure a specific guarantee about his future before making any deal with Mr Johnson and should “not concede any ground”.
In a further fracturing of the Vote Leave campaign, energy minister Andrea Leadsom announced she was making her own bid for the leadership while the Leader of the Commons Chris Grayling has come out for Mrs May.
Launching her leadership bid, Mrs May said the country needed “strong leadership” at a time of economic and political uncertainty and - in a clear swipe at Mr Johnson - warned that politics was not a “game”.
‘If you are from an ordinary working class family, life is just much harder than many people in politics realise,’ she said.
‘Frankly, not everybody in Westminster understands what it’s like to live like this and some need to be told that it isn’t a game. It’s a serious business that has real consequences for people’s lives.’
Mrs May - who was a low-key supporter of Remain during the referendum - made clear she will not attempt to back away from last week’s vote to leave the EU, saying “Brexit means Brexit”.
In a further olive branch to Leave supporters, she said she would create a new Government department, headed by a Cabinet-level minister who had campaigned for Leave, to oversee the UK’s departure from the EU.
Nicky Morgan and Jeremy Hunt both ruled out leadership bids, with Mrs Morgan backing Mr Gove and Mr Hunt backing Mrs May.
In a statement, Mrs Morgan said: “Over the past week it has become clear just how divided our nation is - between young and old, north and south and those with different education and work backgrounds. So it is clear to me that the next leader of the Conservative Party must be someone who can unite the country, who can heal the wounds that the referendum exposed but also, and I think this is what we must not lose sight of, present a confident, positive and optimistic platform for our country’s future in which this Party appeals to the centre ground of British politics.
‘It is equally important that we now secure the right deal for Britain - and the next leader must have the skill and credibility to put together the right team to renegotiate our exit from and future relationship with Europe and explain the final terms to the British people.
‘Having spent the past week talking to colleagues in Parliament, party members and constituents I have concluded unequivocally that the right person to do that is Michael Gove and I am delighted to be endorsing his candidacy today.’
Justice Minister Dominic Raab, a Gove ally who backed the joint ticket and who penned a newspaper article which appeared this morning backing Mr Johnson as PM, said the ex-London mayor’s “cavalier” attitude had scuppered the plan.
The party needed a “unity figure who can bring people together and ultimately put a team together”, he told Sky News.
‘Until the 11th hour Michael Gove was committed to doing that with Boris. It hasn’t happened and some of the reassurances that we had had about turning a dream ticket into a dream team didn’t materialise.
‘One of the key things we wanted to see - people like me who signed up to the Boris/Gove ticket because Michael was felt to be something of a guarantor - is that we wanted to have some principles on which we stood but also anchor a team around Boris to channel that enthusiasm.
‘Despite our best efforts we have not managed to achieve that and the reason was ultimately because Boris didn’t come through and was cavalier with the reassurances he made.’