Boris Johnson is set to be interviewed personally as a witness to several lockdown-busting gatherings in No 10.
Whitehall enforcer Sue Gray is expected to ask the Prime Minister about his role in the gatherings as part of her investigation.
The top civil servant is understood to be looking into at least six social gatherings that took place in Downing Street and across Whitehall in 2020.
Sources suggested that Ms Gray was likely to interview Mr Johnson, who was believed to be present at four of the gatherings under investigation.
One source said: "She wouldn't want to be heading up any investigation that looked like it was ducking speaking to any potential witnesses. It could raise eyebrows if she doesn't."
Mr Johnson has been buffeted by the explosive claims of social gatherings, first revealed by the Daily Mirror. Any suggestion that he personally broke the rules would be deeply damaging.
It came after a bombshell email showed that Downing Street staff were invited by one of the PM's top officials to a drinks party in the No 10 garden during the first Covid lockdown.
The message suggested they should "make the most of the lovely weather " even though the rest of the country was banned from meeting more than one person outdoors.
An email providing evidence of a party on May 20, 2020, when the rest of the country was banned from meeting more than one other person outdoors
Witnesses said about 40 people attended, including Mr Johnson and his wife Carrie, and there were long tables laden with drink, crisps, sausage rolls and other picnic food.
The email represents the first evidence of the social gathering on May 20, 2020, first revealed by ex-No 10 aide Dominic Cummings on Friday.
Labour called for all the evidence to be handed over to the Metropolitan Police which has so far refused to investigate the party claims but is now finally in talks over the Downing Street party 'breaches' after 100-person email invite.
A Met Office spokesperson said: "The Metropolitan Police Service is aware of widespread reporting relating to alleged breaches of the Health Protection Regulations at Downing Street on 20 May 2020 and is in contact with the Cabinet Office.
Just five days after the outdoor gathering, Boris Johnson said police should intervene to stop outdoor gatherings.
Dominic Cummings talked about the garden party last week
POOL/AFP via Getty Images)
And less than an hour before the May 20 gathering, Cabinet Minister Oliver Dowden told the public at the daily No 10 briefing to stick to meeting in pairs outdoors despite the sunshine.
Mr Johnson on Monday repeatedly failed to deny attending the "BYOB" drinks party in the Downing Street garden.
The PM dodged questions over the event on a visit to a vaccination clinic after claims he was at the event, described by a source as a "proper booze up" with wife Carrie.
The email, revealed by ITV News, was sent by the PM's Principal Private Secretary Martin Reynolds to over 100 staff in No 10 including aides, speechwriters and door staff.
Oliver Dowden at the No10 press conference on May 20, 2020
The senior civil servant wrote: "Hi all. After what has been an incredibly busy period it would be nice to make the most of the lovely weather and have some socially distanced drinks in the No10 garden this evening. Please join us from 6pm and bring your own booze!"
Asked on Monday if he and his wife Carrie attended the May 20 event, Mr Johnson repeatedly told reporters: "All that, as you know, is the subject of a proper investigation by Sue Gray."
Just five days after the gathering, the PM himself said police should intervene to stop people having outdoor parties.
Claire from Harpenden' asked Mr Johnson at the daily No 10 briefing how people gathering outside in large groups could be discouraged from breaking the rules.
Boris Johnson pictured during a visit to a Boots pharmacy as he prepared to watch a man receive his Covid-19 booster jab
He replied: "The only reason we have been able to make as much progress as we have, the only reason I am able to announce that we are finally able to begin getting schools back, to begin getting retail back from the June 1, is because this country has observed the social distancing rules.
"So, Claire, feel free to speak to people yourself if you feel that they are not obeying the rules. But the police will step in if necessary and encourage people to obey the law."
The gathering is separate from one on May 15, 2020, of which a photo showed Mr Johnson and his wife sitting with No 10 staff on the terrace with a bottle of wine and cheese.
Covid legal expert Adam Wagner said the event looked "unlikely to be legal for attendees".
"Being outside the home was illegal at the time unless (the only potentially relevant exception) it was for the need to work," Mr Wagner posted on social media.
"'Socially distanced drinks' / 'BYOB' don't sound like work."
Deputy Labour leader Angela Rayner last night told Sky News: "If the PM knew about this party, attended this party and all the other evidence and the parties that are alleged to have happened – all of that evidence – if the law has been broken – should be handed over to the police.
"Nobody is above the law in this country and they should be dealt with appropriately."
Labour MP Neil Coyle called for Scotland Yard to investigate, saying: "The Met cannot keep ignoring the Downing Street transgressions any longer.
"Whilst most of us adapted to restrictions under the pandemic, there was endemic rule breaking at the very top of this Government and prosecutions must follow."