Britain's Prime Minister Boris Johnson reacts during a press conference at Downing Street in London. File/AP

British Prime Minister Boris Johnson will have to fight on Wednesday to defend his premiership after it was revealed a "bring your own booze" gathering was held at his official residence during the first coronavirus lockdown.

Opposition lawmakers have called for Johnson to resign and some in his own Conservative Party have said he should quit if he is found to have broken strict laws his government brought in to prevent the spread of the virus.

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London's Metropolitan Police said they were in contact with the Cabinet Office about the May 2020 gathering in the garden of Johnson's Downing Street residence and office, raising the possibility of a more serious, criminal probe.

A video grab from footage shows the ongoing session of the House of Commons in London on Tuesday. AFP

More than 100 people reportedly received an emailed invitation to the drinks party, at which guests, allegedly including Johnson and his wife Carrie, were encouraged to "bring your own booze".

Johnson, who swept to power in 2019 on a promise to secure Britain's exit from the European Union, has so far refused to say whether he attended the gathering in Downing Street on May 20, 2020.

"His survival is in the balance at the moment," said one senior Conservative lawmaker, speaking on condition of anonymity due to the sensitivity of the situation.

"He has got to make a clean breast. I usually find that if you make your apologies, it defuses the issue. Can't have it hanging on," said the lawmaker.

At the time, the government was ordering ordinary members of the public not to meet, even outdoors, and tight restrictions were in place on social mixing, including at funerals.

Britain's Labour Party deputy leader Angela Rayner speaks in the House of Commons in London.

"It truly is beyond belief that while the rest of the country was in lockdown, Johnson and his staff felt it was acceptable to have a party," Rivka Gottlieb, of the Covid-19 Bereaved Families for Justice group, told Sky News.

British media and social networks were filled with recollections from people who said they stuck to the rules, even as family and friends were sick or dying from Covid — and fury at the apparent double standards.

Police at the time fined those breaching the rules, and had the option to prosecute repeat or egregious offenders.

The main opposition Labour party's leader Keir Starmer accused Johnson of "lying to the British public" while deputy leader Angela Rayner said he was dodging questions that went to the heart of his "honour and integrity".