He was left fighting for his political survival earlier this year after a number of lawmakers from his ruling Conservative Party withdrew their support for his leadership.
Bereaved families of victims of the Covid pandemic also called on Johnson to resign.
"It's now indisputable that whilst bereaved families were unable to be at their loved ones' sides in their last moments, or stood at their funerals alone, the people responsible for protecting us in Downing Street were partying and rule breaking en masse," said Matt Fowler, co-founder of Covid-19 Bereaved Families for Justice.
"It's plain as day that there was a culture of boozing and rule breaching at the highest level of government, whilst the British public was making unimaginable sacrifices to protect their loved ones and communities."
London police are investigating claims that Johnson and government officials organised and attended at least a dozen boozy events in 2020 and 2021 that violated Britain's then-strict virus curbs.
"The investigation into allegations of breaches of COVID-19 regulations in Whitehall and Downing Street continues to progress," the Met said in a statement on Tuesday.
"We are making every effort to progress this investigation at speed. This includes continuing to assess significant amounts of investigative material," it added.
Johnson has already apologised for the parties, which included Christmas celebrations and a drink-fuelled gathering the evening before Prince Philip's funeral.
The prime minister, who initially denied any rule-breaking events had occurred in the complex where he lives and works, has consistently rejected personal wrongdoing.
But his opponents accused him of misleading parliament by insisting the Downing Street events were work-related and within the rules.
And the 57-year-old now faces renewed calls to explain why he attended social gatherings when his government was telling the public that they were illegal.
His office confirmed in February that Johnson had submitted his response to a police questionnaire on the matter but sources said he has not been interviewed in person by investigating officers.
Russia's invasion of Ukraine had eased the political pressure on Johnson, with the international crisis replacing "Partygate" in the daily headlines.
Johnson has sought to play a prominent role in the West's response to the war, hosting near-daily calls with Ukrainian President Volodymyr Zelensky and sending UK military aid to the eastern European country.