It is arguably a measure of the resilience of British democracy that London’s Metropolitan Police Service has opened an investigation into possible Covid-19 lockdown breaches at Prime Minister Boris Johnson’s Downing Street residence after receiving evidence from an internal government probe into a series of gatherings.

After initially refusing to probe the allegations, the UK’s top police officer Cressida Dick told the London Assembly, the capital’s local government council, on Tuesday that her office would launch an inquiry into “a number of events” at Downing Street and Whitehall. Mr Johnson promised to cooperate with police in any formal probe.

“I welcome the Met’s decision to conduct its own investigation because I believe this will give the public the clarity it needs and help draw a line under the matter,” he told parliament. As he lunges from crisis to crisis, the Prime Minister is facing calls to resign amid revelations that he and his staff attended a series of parties during the spring and winter of 2020 when most social gatherings were banned, forcing citizens to miss weddings, funerals, and birthdays, while friends and relatives died alone in hospitals.

Such gatherings are already being investigated by a senior civil servant, Sue Gray, whose report will be crucial in determining whether Mr Johnson can remain in power. The Cabinet Office said Gray’s investigation would continue. But it was not immediately clear whether she would have to delay the announcement of her findings because of the police investigation.

Mr Johnson has apologised for attending a party in the garden of his Downing Street offices in May 2020 but said he had considered it a work gathering that fell within the social distancing rules in place at the time. In the latest revelation, ITV News reported late on Monday that Johnson attended a birthday party in his Downing Street office and later hosted friends at his residence upstairs in June 2020.

The Mayor of London, Sadiq Khan, welcomed the police investigation, saying “no one is above the law”. “The public rightly expect the police to uphold the law without fear or favour, no matter who that involves, and I have been clear that members of the public must be able to expect the highest standards from everyone, including the Prime Minister and those around him,” he said.

The Deputy Leader of the Labour Party, Angela Rayner, renewed opposition calls for Mr Johnson to resign. “Boris Johnson is a national distraction. Conservative MPs should stop propping him up and he should finally do the decent thing and resign.”

Police have previously faced criticism for suggesting that they would not investigate the “partygate” scandal because they do not routinely investigate historical breaches of coronavirus regulations. But an investigation was warranted in this case because there is evidence that those involved knew or should have known what they were doing was illegal.