British Prime Minister Boris Johnson is on Wednesday preparing for the release of the long-anticipated partygate report into law-breaking parties at Downing Street, compiled by senior civil servant Sue Gray.

Gray's interim report, critical of leadership failures, had triggered a police investigation under Scotland Yard's Operation Hillman. That investigation concluded last week with 83 fines being issued for breach of coronavirus lockdown rules within UK government quarters, including one each for Johnson, wife Carrie, and UK Chancellor Rishi Sunak.

It cleared the decks for the original Sue Gray report to be published in full, which is expected to be first handed over to Downing Street before Johnson addresses Parliament over its findings.

"It's not just the optics, it's understandable that people will feel anger because there were instances where people couldn't attend funerals of close friends, couldn't visit loved ones," UK Environment Secretary George Eustice told the BBC, with reference to the breaches.

As well as his appearance in the House of Commons, Boris Johnson is also set to face a tough meeting of the powerful Tory backbench 1922 committee. After an initial uproar over partygate, most of his party MPs have fallen behind the party line against the backdrop of a soaring cost of living crisis and Russia-Ukraine conflict.

However, the issue has been reignited as the BBC's Panorama' programme aired footage of Downing Street insiders speaking anonymously of a boozy culture throughout the lockdown.

Three insiders told the documentary of chaotic, crowded gatherings at the heart of government in which people sat on others' laps in complete disregard of the social distancing rules and rooms were left strewn with bottles of alcohol. One of the BBC's sources said they felt such "parties" were licensed by the UK PM as he was there and did not demand it be shut down.

London Mayor Sadiq Khan is among Opposition Labour leaders calling for an explanation from the Metropolitan Police why the UK Prime Minister was only fined for a birthday party in June 2020 and not handed a second fine for another party after pictures emerged of him holding up a glass at a farewell drinks party on November 13, 2020. The rules in force at the time banned indoor gatherings of two or more people, except if "reasonably necessary" for work purposes.

Downing Street has said that Johnson would address Parliament "in full" after Gray published her report into the gatherings.