Boris Johnson's anti-corruption tsar today resigned over the PM's "clear" breaches of the Ministerial Code, saying "it's over".
John Penrose quit his post in a stinging letter as he joined Jeremy Hunt, Angela Richardson and Jesse Norman as the latest Tories to come out against their leader.
Tory MPs are holding a historic vote of no confidence in Boris Johnson from 6pm tonight.
If he loses, he must resign and the Conservative Party will hold a leadership election - a crushing blow just 906 days after he won the Tories' biggest majority since the 1980s.
In his damning letter - written before tonight's vote was called publicly - Mr Penrose said he was tipped over the edge by the PM refusing to explain whether he broke the Ministerial Code in public statements on Partygate.
He wrote: "I’m afraid it wouldn’t be honourable or right for me to remain as your anti-corruption champion after reaching this conclusion, nor for you to remain as Prime Minister either."
Speaking to Sky News, Mr Penrose - who is one-half of a Westminster power couple as the husband of former Test and Trace boss Dido Harding - said: “It’s pretty clear that he has broken the ministerial code in a very material way”.
He added: “I as the anti-corruption tsar can’t carry on in that role unless or until that is addressed.”
Mr Penrose said colleagues were asking themselves "can I look at myself in the mirror in the morning… and feel I’ve done the right thing".
Even if Boris Johnson wins by a slim majority, he said, “it feels like the beginning of the end”… “I think it’s over and it feels now like a question of when, not if.”
This contrasts loyal ally Jacob Rees-Mogg, who today claimed the PM being booed at Platinum Jubilee celebrations was a "mere bagatelle" and said he'd stay even if he won by one vote.
A leadership election would take a few months. It starts with Tory MPs whittling the candidates down to two, followed by hustings to Tories around the country and a vote by the party membership.
Last time in 2019, there were 10 candidates for the Tory leadership - Boris Johnson, Jeremy Hunt, Michael Gove, Sajid Javid, Matt Hancock, Mark Harper, Andrea Leadsom, Esther McVey, Dominic Raab and Rory Stewart.
Mr Hunt - who was the run-off candidate in 2019 - dealt Mr Johnson a major blow this morning by calling for him to go, saying: "We are not offering the integrity, competence and vision necessary to unleash the enormous potential of our country".
But last time, Boris Johnson was the obvious frontrunner. This time, Tory MPs are divided and there is no clear heir to the crown.
With the Prime Minister fighting back - including an address to Tory MPs at 4pm today and a begging letter to them warning voters will not "forgive us" for "focussing on Westminster politics" - it's widely expected he'll win the vote tonight.
If 180 or more MPs back the Prime Minister, he is technically safe from a vote for another year. Nearly 140 MPs are on the government payroll alone. It's a secret ballot so they can vote against him, but this feels unlikely on any wide scale.
It doesn't mean he's out of the woods, though. The 1922 Committee is free to change the rules to force another vote in six months - something MPs have discussed before.
More importantly, even without a rule change, having 100 or more of his MPs vote against him would start a clock ticking against the Prime Minister.
Theresa May announced her resignation just 163 days after she won her vote 200-117. And if he clings on he could be defeated at a general election, like John Major was in 1997.
