Boris Johnson said he was "furious" after a video of Downing Street staff joking about an alleged Christmas party emerged and pledged there would "disciplinary action for all those involved" if the allegations were true.

The prime minister apologised "unreservedly" after footage of the former press secretary Allegra Stratton and other political staffers laughing about the alleged party during a rehearsal press briefing on December 22 last year.

However he insisted he had been repeatedly assured that “there was no party and that no Covid rules were broken”.

At Prime Minister's Questions today, Mr Johnson told MPs: “I understand and share the anger up and down the country at seeing Number 10 staff seeming to make light of lockdown measures, and I can understand how infuriating it must be to think that people who have been setting the rules have not been following the rules because I was also furious to see that clip.

“I apologise unreservedly for the offence that it has caused up and down the country and I apologise for the impression that it gives.

“But I repeat that I have been repeatedly assured since these allegations emerged that there was no party and that no Covid rules were broken, and that is what I have been repeatedly assured.

“I have asked the Cabinet Secretary to establish all the facts and to report back as soon as possible – and it goes without saying that if those rules were broken then there will be disciplinary action for all those involved.”