Boris Johnson is facing anger from Tory MPs after he hastily triggered a switch to his Plan B for coronavirus restrictions to tackle the Omicron variant as Downing Street was engulfed in a crisis over allegations of a rule-breaking Christmas party.

The Prime Minister announced on Wednesday that work-from-home guidance will return, Covid health certificates are to become mandatory in large venues and mask rules will be extended to combat the rapidly-spreading strain in England.

But despite the strengthening of the rules he said Christmas parties and nativities could go ahead, as he set out the move while under increasing pressure over the event in No 10 last December.

Conservative backbenchers openly questioned the Government’s “credibility” in being able to enforce the rules while so many believed Downing Street had breached them.

Mandatory mask wearing will be extended to indoor public venues including cinemas, theatres and places of worship from Friday but will not be required in pubs and restaurants, while the guidance to work from home where possible will return on Monday.

The NHS Covid pass, which can be obtained by having two vaccines or a negative lateral flow test, will be introduced for entry into nightclubs and other large venues from December 15, as Mr Johnson set out the “proportionate and responsible” measures.

But he was forced to insist the public understands the “vital importance” of the measures as he faced continued questions over events in the run up to last Christmas and Downing Street’s denials that the gathering on December 18 amounted to a party.

Announcing the restrictions simultaneously in the Commons, Health Secretary Sajid Javid was told to “resign” by senior Tory William Wragg as the party row added to anger and fears that public support for the Conservatives had further “taken a hit” following the Owen Paterson affair.

Conservative former chief whip Mark Harper added: “What I am really concerned about is that it is unquestionably the case that over the last few weeks the Government’s credibility, whether it is on Paterson or on the Christmas parties, has taken a hit.

“Why should people at home listening to the Prime Minster and the Secretary of State do things that people working in Number 10 Downing Street are not prepared to do?”

Tory MP Ben Bradley ruled out voting for Plan B which includes “discriminatory” vaccine passports, and former minister David Davis questioned “how are you going to prosecute people who don’t obey it given the four previous parties?”

“I think the real issue is on the authority of the Government to enforce a, as it were, a new lockdown because people look at this and say why should we? It’s them and us again,” Mr Davis told ITV’s Peston.

At the Downing Street briefing, Mr Johnson denied suspicions he had brought forward the announcement to divert attention from the row and the leaked video showing No 10 staff laughing about restrictions after the alleged party, which forced the resignation of Government aide Allegra Stratton.

“You’ve got to act to protect public health when you’ve got the clear evidence,” the Prime Minister said.

Mr Javid said the new restrictions were being imposed “with a heavy heart” but insisted they were necessary as he said there are 568 cases of Omicron confirmed in the UK, but that the true figure is estimated to be “probably closer to 10,000”.

He said they would be reviewed on January 5, before their expiry date of January 26.

The Scientific Advisory Group for Emergencies (Sage) warned Omicron is spreading “rapidly” and it is possible hospital admissions from the new variant in England could exceed 1,000 per day – and still be increasing – by the end of the year.

“The overall scale of any wave of hospitalisations without interventions is highly uncertain, but the peak could reach several times this level,” the minutes from a meeting held on Tuesday said.

Kate Nicholls, the head of trade body UKHospitality, warned of “catastrophic” results without Government support for the sector, while unions called for furlough measures to return.

Frances O’Grady, the general secretary of the Trades Union Congress, said: “Requiring people to work from home over the busy Christmas period will hit jobs – unless ministers bring back furlough.”

Covid health certificates will apply to unseated indoor venues with more than 500 attendees, and outside where there are more than 4,000 people.

The Prime Minister said passes can be obtained with a negative lateral flow test or by having had two doses of a vaccine, but hinted this could change by saying “we will keep this under review as the boosters roll out”.

MPs are expected to be given a vote on the measures on Tuesday, during which a number of Tory rebels who have been angered by restrictions are poised to oppose the Government.

But shadow health secretary Wes Streeting said Labour would “put public health first” and support the measures, meaning they will almost certainly be approved in the Commons, as he accused the Prime Minister of having “undermined public trust and confidence” at a “critical time”.

“It’s quite clear Boris Johnson simply isn’t up to the job and Conservative MPs need to ask themselves whether they are happy for this man to continue being our Prime Minister,” he told reporters.