Theresa May has privately vowed to thwart any attempt by Boris Johnson to take the UK out of the EU without a deal, her allies have told The Mail on Sunday.
The disclosure comes as senior party figures told The Mail on Sunday that Mrs May had voted for ultra-Remainer Rory Stewart in Thursday's ballot of MPs, which led to a landslide victory for Mr Johnson.
Mrs May, who has vowed to stay on as an MP after she leaves Downing Street next month, has suggested she would join forces with pro-Remain Ministers such as Chancellor Philip Hammond and Pensions Secretary Amber Rudd to try to stop Mr Johnson from leaving after the October 31 deadline 'Deal or No Deal'.
No one ever looked this pleased to see Theresa! Pictured: Former Foreign Secretary Boris Johnson meets association activist to National Conservative Convention Hustings as part of his leadership campaign
Britain's Prime Minister Theresa May arrives at the Houses of Parliament in central London on Thursday June 13
The scale of Mr Johnson's victory means that, barring a major Boris blow-up, the race has effectively turned into a battle for second place, with his vanquished opponents deciding whether to fight on or clamber aboard his bandwagon.
It is predicted that the Commons contest to whittle down the field of six to a final two will conclude as soon as Wednesday, after two more rounds of votes.
Former Tory leadership contender Esther McVey has come out in support of Boris Johnson.
Writing in The Sunday Telegraph, Ms McVey - who was eliminated in the first ballot of MPs - said Mr Johnson had agreed to back her agenda for 'blue collar Conservatism'.
'He has promised to deliver Brexit on October 31, deal or no deal, and has shown time and time again that he is a dynamic leader, capable of building a strong team around him that will deliver on his promises,' she wrote.
'Our country is crying out for strong, optimistic leadership and Boris is the man best equipped to take us out of the EU, to transform our country into an outward-looking, confident, self-governing nation, and to implement a policy agenda that will bring back the voters we have lost and ensure we don't allow Jeremy Corbyn's manifesto of economic carnage to plunge us back into the dark ages.'
The last two will then face an electorate of more than 160,000 Tory members, after a surge of new members who signed up ahead of the contest were deemed eligible to vote by party chiefs.
Last night Mr Johnson moved to scotch suggestions he wished to avoid a run-off, saying it was 'full steam ahead to a contest'.
Esther McVey, who came last with nine votes on Thursday, was the first former candidate to declare her intentions, saying that she would swing behind Mr Johnson and urging her eight supporters to do the same.
A source said: 'Boris has told Esther that he will give his backing to blue-collar Conservative policies, such as the need to invest in the Midlands and North'.
Health Secretary Matt Hancock, who pulled out on Friday, was last night on the brink of declaring, with friends saying that he was wavering