Conservative MPs are to cast their votes in the first ballot in the Tory leadership race after Boris Johnson threw his weight behind fellow Leave campaigner Andrea Leadsom.
The former London mayor - who saw his own leadership hopes dashed after a devastating personal attack by Michael Gove - said she had "the zap, the drive and the determination" to lead the country.
His intervention came as a significant boost to Mrs Leadsom, the energy minister, after she was said to have performed poorly in front of Tory MPs at a leadership hustings at Westminster on Monday evening.
It was widely seen as an act of revenge against Mr Gove, the Justice Secretary, who had been expected to back Mr Johnson for the leadership after campaigning alongside him in the referendum, only to say he was not up to being prime minister.
There was speculation that it could finally scupper Mr Gove's own leadership bid, with many Tories unhappy at the way he treated his erstwhile comrade.
Mr Johnson pointedly praised Mrs Leadsom as being "level-headed, kind, trustworthy".
Ahead of the first ballot, a poll by the ConservativeHome website of 1,214 party members gave Mrs Leadsom a narrow lead with 38%, one point ahead of Home secretary Theresa May - widely seen as the frontrunner - on 37%.
In contrast a YouGov poll for The Times of 994 Conservative Party members suggested that if it came to a final round run-off between the two women candidates, Mrs May would win by by 63% to 31%.
At Monday's hustings, Mrs May strongly defended her insistence that the status of EU nationals living in the UK must be part of the Brexit negotiations after a furious backlash from Tory MPs.
Conservative MPs lined up in the Commons to condemn her comments, accusing Mrs May - seen as the frontrunner in the race to succeed David Cameron - of a "catastrophic error of judgment".
However she took on the issue head-on, insisting the Government could not afford to "give away" its negotiating position and that it had to be able to defend the position of British nationals living in the EU.
"It is about a very logical, correct, practical procedure," a spokesman said.
Mrs May sought further to burnish her leadership credentials calling for MPs to be given a vote on the renewal of Britain's Trident nuclear deterrent before Parliament breaks for the summer.
With Labour in disarray on the issue, she said the Government should "get on" with the job of building a new submarine fleet.
"While it is true that the terrorist threat we face has grown more serious, it does not mean we no longer face a threat from conventional enemies in the forms of other nation states," she told The Sun.
"The House of Commons should, before the summer recess, vote on Britain's next-generation nuclear deterrent - and we should get on with getting it built."
Mrs May goes into the first round of voting with a clear lead among MPs but the contest will be decided by grassroots members throughout the country.
She is joined on the ballot paper by Mrs Leadsom, Mr Gove, Work and Pensions Secretary Stephen Crabb and former defence secretary Liam Fox.
The candidate who finishes last will be eliminated and others may choose to drop out if they feel they have no chance of succeeding.
Further votes of MPs will be held - with the next due on Thursday - until the candidates are whittled down to a shortlist of two who will go forward into the final postal ballot of the entire party membership.