A top civil servant investigating alleged law-breaking in Downing Street has found a email warning Boris Johnson's principal private secretary against holding a lockdown-breaching party in Number 10, ITV News understands.
The discovery by Sue Gray, who has been appointed by the PM to investigate numerous allegations of illegal behaviour in the heart of government, was revealed by ITV News Political Editor Robert Peston.
He said it leaves a "huge question" hanging over Mr Johnson's claim he hadn't been warned the May 20, 2020 party was against the rules and that he only attended because he thought it was a work event.
The warning email - sent by a senior official - told Mr Johnson's principal private secretary Martin Reynolds, the party "should be cancelled because it broke the rules", according to Peston.
What is the email Sue Gray has found and what does it mean for her investigation?
The official has told Peston he also "probably" personally warned the PM against the party "but I honestly can’t remember".
Claims the party went ahead with the PM and his wife in attendance were first made by Mr Johnson's former top aide Dominic Cummings, who wrote in a blog post that he also personally warned his boss against the party.
Reports of the party were substantiated when ITV News released a leaked email, sent by Mr Reynolds, inviting more than 100 staff to "make the most of lovely weather" by attending "socially distanced drinks" in the Number 10 garden.
Mr Cummings wrote Mr Reynolds "checked with the PM whether the party should go ahead. The PM agreed it should".
Mr Cummings also said the PM dismissed his concerns over Mr Reynolds sending the invite.
Mr Johnson has been accused of misleading Parliament with his excuse for attending the party and is facing calls to resign from his own MPs over the debacle.
Even a minister on Wednesday admitted "millions of people" don't believe the PM's explanation, however Mr Johnson has repeatedly urged people to wait for Ms Gray's investigation to conclude before passing judgement.
The PM resisted intense pressure to quit at PMQs amid a chorus of calls for his resignation, which included a intervention from former Cabinet minister David Davis who told Mr Johnson: "In the name of God, go."
Peston says the warning email found by Ms Gray is "essential evidence" that will help Ms Gray determine whether Mr Johnson knowingly misled Parliament with his "work event" explanation.
Ms Gray is expected to give the PM a report on the conclusion of her investigation next week.
If he is found to have lied to MPs it will be hugely damaging for the PM, with more than 20 letters of no confidence in his leadership already submitted.