Number 10 has repeatedly refused to deny there was a second party on Boris Johnson's 56th birthday, held in his Downing Street flat hours after a celebration earlier in the day which saw him, his wife and the chancellor all fined.
Reports say Carrie Johnson hosted an unspecified number of male friends in the flat during the UK's first lockdown on June 19, 2020 when indoor gatherings of people from different households were banned.
Text messages reported by the Sunday Times and the Telegraph suggest the prime minister may also have attended.
The revelations are particularly damaging for the prime minister because the party was not mentioned in Sue Gray's 37-page report into coronavirus-rule-breaking on Downing Street.
The prime minister's deputy spokesman did not deny there was a gathering as reported, suggesting the civil servant investigating parties would have looked into the alleged gathering if there was "credible" evidence of law-breaking.
"I have seen the same reporting that you have but I think this is covered in the terms of reference in Sue Gray's report where she clearly said that any other gatherings ... where she received credible allegations, would be looked into.
"Downing Street staff were given clear guidance to retain any relevant information and co-operate fully with the investigation."
The Sunday Times reported on text messages, which have since been passed to Cabinet Secretary Simon Case, that showed a Number 10 aide alerted Mrs Johnson to the fact her husband was on her way up to the flat.
Reports say she told the aide she was already in the flat with friends. According to the Telegraph, she said: “Great. I am already here with the gays,” - an affectionate term she reportedly uses to refer to her gay male friends.
A spokeswoman for Carrie Johnson told The Times: "Sue Gray was aware of these exchanges as part of her exhaustive inquiry into alleged breaches."Staff were given ample opportunity to present evidence including these messages and all relevant information was passed to the Metropolitan Police for investigation.
"The lunchtime gathering in the Cabinet Room on 19 June 2020 was subsequently found to be in breach and a fixed penalty notice was issued to Mrs Johnson among others for which she apologised unreservedly and promptly paid the fine."
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Labour Deputy Leader Angela Rayner said: "It appears that Number 10 has now stopped denying that another lockdown-breaking secret gathering took place in the Downing Street flat.
“Less than a week after the release of the Gray Report, this raises serious questions about whether Downing Street has been caught lying yet again and why the event has not been investigated.
"The prime minister must come clean with the British people.”
It comes as the number of Tory MPs to have publicly declared they have submitted letters of no confidence in the prime minister continued to tick upwards.
Conservative former minister Jeremy Wright is the latest to urge a resignation after releasing a statement on Monday morning.
He followed former health minister Steve Brine and Anne Marie Morris who called for the prime minister to quit over the weekend.
Sir Graham Brady, chairman of the 1922 Committee of backbench Tories, will be obliged to order a confidence vote if he receives 54 letters demanding one.
More than 20 MPs have publicly stated that they no longer support Mr Johnson but the actual number of letter writers may be higher given it is a secret process, with Sir Graham not declaring how many have been received until the threshold has been met.