Boris Johnson has dodged a question on whether he attended a party in Downing Street in May 2020 when England was in the first national lockdown.
The Sunday Times cited three sources stating Mr Johnson's principal private secretary Martin Reynolds emailed officials with an invite to the event on 20 May suggesting attendees "BYOB" (bring your own bottle, or booze).
The newspaper alleged that the PM attended the party in Number 10's garden with his wife, Carrie Johnson.
Outdoor gatherings were banned in England at the time under the government's coronavirus rules.
When asked by a reporter whether he and his wife attended the event, the PM refused to say.
Mr Johnson instead insisted it was a matter for Sue Gray, the senior official leading an investigation into reports of lockdown-busting parties across Whitehall.
"All that, as you know, is the subject of a proper investigation by Sue Gray," he said.
On whether he had been interviewed by Ms Gray, he said: "All that is a subject for investigation by Sue Gray."
Starmer: PM has 'lost moral authority'
Sir Keir Starmer said the PM would have "serious questions to answer" if he was found to have attended the event.
Asked whether Mr Johnson would have to resign if he was found to have attended a party during lockdown, the Labour leader said: "We need to let the inquiry take its course, see what the findings are.
"The prime minister has insisted he broke no rules, so if the finding is that he did, then he will obviously have very serious questions to answer.
"Let's let the inquiry play out, let's see what the findings are, and then go from there."
Sir Keir added: "The prime minister has lost huge authority with the public because of these allegations of parties in Downing Street.
"To stand at a press conference, instructing the country to comply with restrictions - which really impacted families across the country - whilst at the same time there's emerging evidence of parties in Downing Street does diminish his authority, his moral authority, to ask others to comply with those rules."
Meanwhile, Downing Street has denied reports that Mr Reynolds is to be moved from his role to another post following the Sunday Times' claims.
"The prime minister has full confidence in his team. There is no change in that post," the PM's spokesperson said.
The spokesman also refused to be drawn on reports that Mr Johnson had attended the event, again referring to the ongoing investigation by Ms Gray.
Ms Gray took over the investigation into the alleged Downing Street Christmas parties after Cabinet Secretary Simon Case quit his role leading the inquiry.
Mr Case faced allegations that his department's office held its own gathering for a partially virtual quiz on 17 December last year.
The PM has repeatedly denied personally breaking any COVID rules and has said the results of the inquiry into the parties would be released "as soon as we reasonably can".