Boris Johnson and Rishi Sunak were locked in secret talks on Saturday night to strike a deal over the next Prime Minister.
The duo arranged to chat at about 3.30pm, but it was unexpectedly pushed back to 5.30pm, then around 9pm, according to reports.
A senior Tory told the Sunday Times it was "like getting pandas to mate". The talks were said to have finally broken up after 11pm.
Multiple sources close to the two men refused to comment on the talks and details were scarce as dawn broke on Sunday.
Boris Johnson backers initially fuelled speculation of a deal after a string of former allies abandoned him to back Mr Sunak instead.
Supporter John Whittingdale told Times Radio: "If an agreement is reached, then I will be very happy and pleased to support it." Former chief whip Chris Heaton-Harris added: "If they came together, I think it would be a dramatic turn for the better for us all."
But in a bombshell intervention, Boris Johnson told backers on an 8am call that there was no deal and he would not back down. And minutes later Rishi Sunak formally declared his bid for leader.
Johnson supporter Jacob Rees-Mogg appeared to confirm this, telling the BBC: "I have been speaking to Boris Johnson, and clearly he's going to stand, there's a great deal of support for him".
And ex-Chancellor Nadhim Zahawi suggested Boris Johnson was still fighting on by backing him at 9.30am. He claimed the ex-PM "got the big calls right" and "Britain needs him back. We need to unite to deliver on our manifesto."
The two Tory MPs are the frontrunners in the race to lead their party, and ultimately become the new PM.
Rishi Sunak has more support among MPs but Boris Johnson's backers believe he would win if it goes to a vote of party members.
But there is huge Tory pressure on Mr Johnson to drop out, "crown" Mr Sunak and avoid more Tory splits.
Mr Sunak has already reached the milestone of 100 publicly-backing Tory MPs he needs to get on the ballot. Boris Johnson's team has been accused of "desperate bluffing" after claiming to have reached 100 - despite fewer than 60 public pledges.
A string of Tories have lined up to warn Boris Johnson not to stand for PM - with his former deputy PM, chief of staff and Brexit chief all turning against him to back Rishi Sunak instead.
That was a blow for the disgraced ex-PM as he flew in to Britain from a luxury Dominican Republic holiday on Saturday morning.
Steve Baker, an influential right-wing Brexiteer who backed Mr Johnson in 2019 and Liz Truss this year, launched a blistering attack on the former premier's comeback bid and backed Rishi Sunak instead.
Speaking on Sky News, Mr Baker said Mr Johnson "is a guaranteed nailed-on failure” and "he’s bound to implode, taking the whole government down with him”.
“Boris Johnson would be a guaranteed disaster,” the Northern Ireland minister added, and "I'm not willing to lay down my integrity for Boris Johnson".
Their rocky past could set up a grim final battle for a place in Downing Street.
Mr Sunak was previously Mr Johnson's Chancellor of the Exchequer when he was Prime Minister, taking the helm after Sajid Javid quit and being plunged into running the furlough scheme at the start of the 2020 lockdown.
Although the programme was largely successful, it was also the Chancellor's idea to bring the divisive 'Eat Out to Help Out' scheme.
Their relationship ultimately ended in dramatic circumstances, when Mr Sunak resigned from his role in Mr Johnson's Government earlier this year.
His resignation - along with Health Secretary Sajid Javid - sparked a mass exodus from the Cabinet. Mr Johnson was forced to resign from his role as PM after three years, and the bad blood between the duo has rumbled on ever since.
Earlier this week, head of the 1922 Committee Sir Graham Brady revealed that any MP hoping to become the Conservative Party leader would need at least 100 nominations from their fellow MPs.
Neither MP has formally announced their decision to run for PM, fuelling speculation of a deal. They likely agreed to a meeting to form a pact moving forward, in a bid to bring the Tory in-fighting to an end, according to The Sun.
If they couldn't reach a deal, it could end up in a straight-up battle between the two MPs.
Mr Johnson would likely win in a ballot of Tory members against Mr Sunak.
But, it's still not entirely clear whether the former PM has the backing of enough MPs to even reach the ballot, despite his entourage's claims.
As of Sunday morning, the only MP to have formally entered the race to become the new PM is Penny Mordaunt. She narrowly missed out on the final two to Liz Truss in July. But she has only a quarter of the public backers she needs by 2pm on Monday.
The idea of a stitch-up behind closed doors has fuelled furious calls for a general election to let the British people decide.
