For Boris Johnson's critics, the evidence could hardly be more damning. The video, taken on 22 December last year, shows his spokeswoman Allegra Stratton holding a practice press conference. "I've just seen reports on Twitter that there was a Downing Street Christmas party on Friday night," says Ed Oldfield, a special adviser playing the part of a journalist. "Do you recognise those reports?"

Stratton first laughs - "I went home" - and then searches for a response: "Hold on, hold on, erm..."  Oldfield presses her: "Would the Prime Minister condoneForgive or overlook. It derives from a Latin verb meaning to favour someone by not punishing them. having a Christmas party?" FlummoxedBewildered. The word is thought to have originated in a local dialect., Stratton laughs again: "What's the answer?" she asks - to which Oldfield replies: "I don't know!" Then other Downing Street staff join in, laughing too: "It wasn't a party," suggests one, "it was cheese and wine."

"This fictional party was a business meeting," says Stratton finally, "and it was not socially distanced."

The party allegedly took place on 18 December, when social gatherings in London were banned. On that date, 489 people died of Covid in hospital, with their families not allowed to see them. The following day, Johnson announced that Christmas was cancelled for the whole country.

That Downing Street broke its own rules is bad enough; for Tory insiders to be laughing about it is, for many, completely unforgivable. And though Johnson is not believed to have been at the party, the idea that he knew nothing about it looks increasingly far-fetched.

Yet in a far from convincing performance at Prime Minister's Questions yesterday, he stuck to his story: "I have been repeatedly assured that there was no party and no breach of Covid rules."

When Johnson won the 2019 general election, his position seemed unassailable. He had a majority of 80 seats and the biggest shareThe Conservatives won 43.6% of the vote. In 1979, Mrs Thatcher won 43.9%. of the vote for 40 years. In April 2020, he had an enviable approval ratingA simple test of popularity that calculates the difference in the percentage of people who approve and disapprove of a politician. of 66% among voters.

But recent events have made him and his government look chaotic, with even diehard Tories doubting him. Johnson is accused of mishandling the pandemic by lifting all restrictions prematurely this summer. A fortnight ago he gave a rambling speech about Peppa Pig.

Thousand of refugees arriving by boat have made a mockery of his immigration policy. His failed attempt to defend Owen Paterson from sleaze allegations was deeply embarrassing. Many of his own MPs rebelled over both that and his proposed changes to social care.

On Monday came revelations of the Foreign Office's blunders during the evacuation of Kabul - including the fact that animals had been given priority over humans.

A Christmas party may seem trivial, but history often turns on small events. Neil KinnockLabour's leader from 1983, he resigned after failing to win the 1992 general election. stumbling on Brighton Beach may have put paid to his chances of becoming prime minister. Gordon Brown calling a Labour voter a "bigotedA bigot is a person who hates or refuses to accept the members of a particular group. woman" was seen as a defining momentBrown's remark was in response to a mention of immigration. It was caught on microphone and was felt to be hugely damaging to his image. in his failed 2010 campaign. A would-be Labour leader, David MilibandHe was defeated in the 2010 leadership election by his brother Ed. He is now the head of a charity, the International Rescue Committee., was haunted by a ridiculous picture of him holding a banana.

Some say, yes: what MPs care about is winning the next election, and the public fury over this incident - added to a worsening pandemic and a looming crash - shows that Johnson would probably lose it. Any Tory leader also needs the backing of the Daily Mail, and its new editor has no faith in him.

Others argue that Johnson has a way of surviving and triumphing against the odds. He overcame great scepticism within his own party to become its leader. In 2019 he helped it gain Labour seats that were thought to be unwinnable. His MPs know that no one else could repeat his success.