At least 30 more fines are to be issued as a result of a police investigation into lockdown-breaking parties in Downing Street and Whitehall.

The Metropolitan Police said that as of today it had made over 50 referrals to the ACRO criminal records office – which is responsible for issuing the fixed penalty notices – for breaches of COVID-19 regulations.

Last month, the Met said it had referred 20 such cases to be issued with fines as part of the investigation, Operation Hillman, which covers events in 2020 and 2021.

The Liberal Democrats said the update exposes the “shocking scale of the criminality in Boris Johnson’s Number 10”.

Downing Street has said it will make public if Boris Johnson or cabinet secretary Simon Case receives a penalty but as yet there have been no such updates.

The prime minister, as well as chancellor Rishi Sunak, were among 100 individuals who were sent formal legal questionnaires related to the investigation.

The Met said: “We are making every effort to progress this investigation at speed.

“This includes continuing to assess significant amounts of investigative material from which further referrals may be made to ACRO.”

Mr Johnson has refused to admit, when pressed by MPs, that the issuing of fines meant that criminality had taken place in Downing Street though two of his cabinet ministers, Dominic Raab and Anne-Marie Trevelyan, have conceded that rules were breached.

Lib Dem leader Sir Ed Davey said: “The police have now completely shredded Johnson’s claims that no laws were broken.

“He cannot be trusted and cannot continue as Prime Minister.

“No other leader in any other organisation would be allowed to continue after law-breaking on this scale.

“If Boris Johnson won’t resign, Conservative MPs must show him the door.”

Police have said they will not identify those who are fined.

However, Helen MacNamara, the government’s former ethics chief, has publicly apologised after receiving a penalty – which related to a leaving party held in the Cabinet Office on 18 June 2020.

The first batch of fines are understood to have related to that gathering as well as events in Downing Street on the night before the Duke of Edinburgh’s funeral in April last year.

Mr Johnson has faced claims that he breached the ministerial code after telling parliament initially that no rules had been broken in relation to the parties.

The prime minister told the Commons liaison committee last month: “I’ve tried my best to be as clear as I can about my understanding of events.”