British ex-premier John Major on Thursday accused Prime Minister Boris Johnson of breaking Covid laws and fostering public "contempt" for politics, as police prepared to grill Downing Street party-goers.

The unusually blunt attack from Major on his Conservative colleague underlined the political jeopardy Johnson faces with several lawmakers from the ruling party calling publicly for his resignation.

Police are now reviewing a December 2020 event in Downing Street, after the Daily Mirror printed a photo of Johnson and two colleagues -- one draped in tinsel -- near an open bottle of sparkling wine.

On a visit to NATO headquarters in Brussels, Johnson was asked if he would resign if he is fined by police for breaching Covid restrictions on social gatherings.

"That process must be completed and I'm looking forward to it being completed, and that's the time to say more on that," he told reporters.

Major, prime minister from 1990 to 1997, stopped short himself of demanding that Johnson step down, but said any other leader found to have deliberately misled parliament would feel compelled to quit.

"At Number 10, the prime minister and officials broke lockdown laws. Brazen excuses were dreamed up," he said in a speech to the Institute for Government think-tank.

Britain's Prime Minister Boris Johnson has come under fire over his government's coronavirus testing plans
Britain's Prime Minister Boris Johnson has come under fire over his government's coronavirus testing plans POOL / Ian Vogler

"Day after day the public was asked to believe the unbelievable. Ministers were sent out to defend the indefensible, making themselves look gullible or foolish as they did so," he said.

"Collectively, this has made the government look distinctly shifty, which has consequences that go far beyond political unpopularity."

Britain's international reputation had taken a blow, Major said, accusing Johnson of undermining peace in Northern Ireland by failing to understand what he signed up for with Brexit.

"Too often ministers have been evasive and the truth has seemed to be optional," he added. "Outright lies breed contempt."

Major is the second of Johnson's Conservative predecessors in the post to criticise him over the "partygate" revelations.

John Major is the second former Conservative prime minister to attack Johnson over 'partygate' revelations
John Major is the second former Conservative prime minister to attack Johnson over 'partygate' revelations POOL via AFP / Chris Jackson

Last week, Theresa May told parliament a civil servant's report into the events showed that Downing Street "was not observing the regulations they had imposed on members of the public".

She asked Johnson to clarify whether he "had not read the rules or didn't understand what they meant and others around him, or they didn't think the rules applied to Number 10".

In response to Major's speech, Johnson's official spokesman said: "Obviously people are free to give their opinions.

"I think what the public wants to judge governments on is what it is delivering for them, how it is changing and improving their lives."

The spokesman added that Johnson had yet to hear from London's Metropolitan Police, which says it intends this week to send out questionnaires to more than 50 individuals in Downing Street.

The questionnaires will ask about their activities on the dates of at least 12 gatherings in Downing Street over 2020 and 2021.

The document "has formal legal status and must be answered truthfully" within seven days, the police noted.

Asked whether the government would confirm if Johnson is one of the recipients, Defence Secretary Ben Wallace told BBC radio: "The prime minister has said he will be full and transparent."

Separately, the opposition Labour party called for the police to investigate how Johnson came to use a wealthy Conservative donor for a lavish redecoration of his Downing Street flat.

Lawyers for Labour wrote to the Met that there was a "reasonable suspicion" that the prime minister had broken anti-bribery laws, and the force was "duty-bound" to investigate.

The police confirmed receipt of the letter, and said the demand was being reviewed.

Downing Street said in response: "These allegations are categorically untrue and a clear misrepresentation of the facts."