British Prime Minister Boris Johnson is alleged to have attended several of the Downing Street gatherings. (Photo: Agencies)

Twenty fines will be issued for breaches of COVID-19 rules during a series of lockdown-breaking parties held at Prime Minister Boris Johnson's offices and residence during the height of the pandemic, British police announced on Tuesday.

They said they would not release or confirm the identities of the people to be fined, in line with approved practice concerning fixed penalties. However, Johnson has already faced renewed calls to resign as a result of the fines.

Scotland Yard said it is investigating 12 gatherings at Downing Street and the Cabinet Office after an internal inquiry found Johnson and his staff had enjoyed alcohol-fuelled parties in and around the prime minister's official residence.

"We will today initially begin to refer 20 fixed penalty notices to be issued for breaches of Covid-19 regulations," the Metropolitan Police said, adding that they would be issued by the ACRO Criminal Records Office.

"We are making every effort to progress this investigation at speed and have completed a number of assessments. However due to the significant amount of investigative material that remains to be assessed, further referrals may be made to ACRO if the evidential threshold is made."

Such fines were issued throughout the UK's lockdown to people judged to have broken coronavirus rules. The fine for attending a gathering of more than 15 people during the height of the pandemic was an 800 pound ($1,048) fine.

The police will not confirm which events the fines at Downing Street referred to as it would potentially help identify the individuals involved in the parties.

Revelations over the Downing Street parties, many of which were held when UK government rules prohibited people from attending funerals or saying farewell to loved ones dying in hospital, prompted widespread anger.

At the height of the scandal, Johnson faced calls to resign from both the opposition and MPs in his own party, with at least 17 Conservative MPs thought to have submitted letters of no confidence in the prime minister. Today opposition parties have repeated their demand that he should go.

British police have also been criticized during the investigation for issuing questionnaires to people thought to have been involved in the parties, instead of asking them to explain their attendance in person. Johnson was among those who received such a questionnaire.

Police said earlier this month they had started to interview witnesses as part of the investigation.