Jeremy Corbyn has again repeated his demand that May leaves office immediately - John McDonnell said he expected Boris' leadership campaign to start today.
The other candidates behind Boris according to the bookies are David Davis, Amber Rudd, Ruth Davidson, Phillip Hammond , Sajid Javid and Michael Gove.
George Osborne said the result was 'completely catastrophic for the Conservatives and Theresa May' and said the woman who sacked him would be lucky to survive.
Nigel Evans, member for Ribble Valley, told BBC 5 Live that her decision to call a snap election meant: "We didn't shoot ourselves in the foot, we shot ourselves in the head".
The bookies have made Boris Johnson favourite to be the next Tory leader - and probably prime minister - followed by Chancellor Philip Hammond and Home Secretary Amber Rudd.
Former Ukip MP Douglas Carswell said he believes that it will be a battle between Boris and Brexit Secretary David Davis followed by a general election later this year.
The former Mayor of London's chances will have been bolstered by Hammond's shaky start as Chancellor.
Amber Rudd only held her Hastings seat by 300 votes and if an election was called again this year there would be a real chance she would be toppled.
Johnson looked in a hurry to be leaving the count at Brunel University, and as soon as all the speeches were over made a dash for the exit.
He stormed into the venue simply stating that it was "early days".
In his victory speech Johnson said: "It is early to comment on the events unfolding tonight in this General Election.
"But one this is absolutely clear, I think to all of us who have been elected as MPs tonight across our fantastic country, that is we have got to listen to our constituents and listen to their concerns."
He added: "I am proud to be re-elected as the member of parliament for Uxbridge and South Ruislip."
Johnson said nothing else about wider results of the night and how his party was performing, but thanked all involved in the election in the borough.
A number of other declarations were made at the same count, including that for Hayes and Harlington which was won by John McDonnell.
McDonnell said Theresa May had made a big mistake in calling the General Election.
Asked if she should resign, he said: 'Increasingly now political commentators and by the sound of it, some Tory MPs now consider her position is now untenable.
"I was expecting Boris Johnson to launch his leadership campaign tonight. But clearly not tonight, maybe tomorrow."
Earlier McDonnell declared Labour would form a government at the first opportunity, even if it was a "minority government."
McDonnell said: "I think Theresa May actually called a General Election despite promising some seven times not to. And people thought this was unnecessary. And she put party before country."
He added: "We will form a government at the first opportunity.
"Whenever we get the opportunity we will do, if it's a minority government we will. But we don't know the results of this election yet.
"We are extremely cautious, let's see what the results are."
The ailing premier is now thought to be with her husband Philip and closest aides Nick Timothy and Fiona Hill.
The PM's joint chiefs of staff became the focus of blame this morning as May was warned she would have to shake up her team if she clings to power.
One Tory minister last night said: "Is it the end of the Nick and Fi show?"
Hill and Timothy have worked with May since she was Home Secretary.
One senior backbencher told the Mail: "Obviously some people are talking about changing personnel. I think that's very much a matter for the Prime Minister to decide for herself.
"If you start changing your advisors then you have conceded something quite important. But I do think there needs to be a real effort to make sure there are open lines of communication."
Before the polls closed last night, one Minister described the Conservative campaign as the "worst in living memory".
They said: "Is it the end of the Nick and Fi show? Everybody wants it to be but she [Theresa May] is very stubborn."
Another backbench Tory added: "There have to be lessons learned. There's a lots of people with grey hair who Theresa could use. If the manifesto had been properly passed by them they would have put a stop to it.
"Manifestos are usually apple pie and ice cream. Instead we got three spoonfuls of arsenic and - surprise surprise - people didn't like it."
Asked if May should resign, Soubry told the BBC: "That's a matter for her. It's bad. I think she in a very difficult place.
"She's a remarkable woman and a very talented woman but she now has to consider her position."
Soubry added: "Theresa did pit her mark on this campaign and she takes responsibility, she always does and I know she will, for the running of the campaign as well.
"It was a tightly knit group and it was her group who ran this campaign"
Criticising the Conservative manifesto, Soubry said: "When you talk about the changes we're going to make to school lunches, you start with the plan that says children from poorer families will now get two free meals a day, you don't start from the basis that some children will lose a free school meal.
"All the way along those of messaging were appalling. And then you had the change of heart on social care deeply flawed Theresa May.
"She didn't look the strong and stable leader Prime Minister and leader she said she was and that was a very difficult and very serious blow in terms of her own credibility."
Conservative MP Nigel Evans told BBC Radio 5 Live: "We didn't shoot ourselves in the foot, we shot ourselves in the head."
Senior Tories were last night speculating openly about whether the Prime Minister would be able to survive.
"It's a disaster," said one. "We may be able to form a government, but it will be a very different government - and it may not be a Theresa May government."
May will stay Prime Minister until she tenders her resignation to the Queen and she cannot do so without recommending a successor.
The rules mean she has the first chance to form a government in the hung parliament if she chooses to carry on.
She could suggest to Labour that the hung parliament means the two parties should work together in a national government to deliver Brexit together.
All of the constitutional rules could be swiftly thrown out if May quits as Tory leader either of her own volition or because party chiefs make clear her position is untenable.
May's mood was "calm" and "sombre" when she addressed staff at Conservative Party HQ - but she "didn't directly refer to her future", BBC political editor Laura Kuenssberg says.
She didn't say she would stay, and the fact she didn't mention this could mean she hasn't made up her mind yet, and "puts the chance of resignation on the table", Laura adds.
As her own result was declared in Maidenhead, May appeared defiant insisting the country needed a 'period of stability.
Katie Perrior, May's former director of communications at 10 Downing Street, said it would be "incredibly difficult" for her former boss to stay as Conservative leader.
She told Sky News: "It depends whether or not the Conservative Party says 'Look, we're 10 days away from the start of the Brexit negotiations, what we need right now is a stable leader to rally around but we will look at this in a month or two and come back to it'.
'But that is whether or not they're willing to do that.
"There are a lot of bruised people out there this morning ... they're bruised because they may have had lower majorities or indeed they've got friends who have lost their seats."
Perrior said it was "damaging" for May not to take part in the televised debates, adding: "I think the communications for the campaign have been pretty awful."
Looking shattered at her Maidenhead count, she said: "At this time more than anything else this country needs a period of stability.
"And if as the indications have shown the Conservative Party has won the most seats and probably the most votes then it will be incumbent upon us to ensure we have that period of stability and that is exactly what we will do."
The bookies opened markets on the next Conservative leader and by 3am Boris Johnson was the favourite at 2/1, Philip Hammond joint second with David Davis at 7/1 and Jeremy Hunt priced at 12/1.
If she does manage to stay on as Prime Minister, she faces being severely weakened going into Brexit talks with EU chiefs, and could be held hostage by the right wing of her party.
Last month, she wrote: "The cold hard fact is that if I lose just six seats I will lose this election and Jeremy Corbyn will be sitting down to negotiate with the presidents, prime ministers and chancellors of Europe."
Critics were predicting May could fail her own test after early results went against her.
After the exit poll was announced, the odds on Boris Johnson becoming the next Prime Minister shortened.
Several of her ministers were expected to lose their seats. One of her manifesto authors, Ben Gummer lost his seat to Labour's Sandy Martin in Ipswich.
Treasury minister Jane Ellison was the first casualty - losing Battersea in South London in a disastrous night for the party in the capital.
Senior figures such as David Cameron's former No 10 spin doctor Sir Craig Oliver openly speculated about May's future.
Attempting to rally support for May, David Gauke, chief secretary to the Treasury, told Sky News that Tory MPs had a "responsibility to continue to support her".
"With all caveats repeated, I think Theresa May continues to be the dominant figure within the Conservative Party. She won the leadership with a massive majority with MPs.
"I think she is not only the best person to lead not just the party but the country and I think that given that we have got really important negotiations in 11 days' time, I think the responsibility of those of us who hope to be elected as Conservative MPs in the next few hours is to continue to support her."
But International Trade Secretary Dr Liam Fox appeared to raise doubts about her future. Asked to guarantee she will not have to resign, he said: "It's very early in the evening and we'll have to wait and see".
May faces a brutal post-mortem into a disastrous campaign which began with the Conservatives boasting double digit poll leads and Mrs May sky-high personal ratings.
