British Prime Minister Boris Johnson and other senior officials "must bear responsibility" for the culture that led to numerous lockdown-breaching parties in Downing Street, said a long-awaited report by a senior civil servant published on Wednesday.

Revelations that Johnson and his staff repeatedly flouted the rules they imposed on others have elicited outrage in Britain and led to calls from opponents for the prime minister to resign. But most lawmakers in his party have so far stood by him, and it is not yet clear if the much-anticipated report will change that.

Johnson said he took full responsibility after the report detailed a series of illegal alcohol-fuelled parties during coronavirus lockdowns.

"I ... am humbled and I have learned a lesson," he told Parliament to jeers from the opposition benches.

Johnson has faced repeated calls to resign from opposition politicians and some in his own party after it was revealed both he and officials had broken the rules that meant people could not socialise outside their households or even, in many cases, attend funerals for loved ones.

The report by senior official Sue Gray did not specifically blame Johnson, but gave graphic details and included photographs from more than a dozen gatherings, some of which he attended.

Johnson was among those fined over a party to celebrate his 56th birthday on June 19, 2020, though Gray said he was unaware of the gathering in advance.

"The events that I investigated were attended by leaders in government," the report, compiled by Sue Gray, said, adding: "Many of these events should not have been allowed to happen."

"The senior leadership at the centre, both political and official, must bear responsibility for this culture."

The 60-page report by Gray, the top civil servant in the Cabinet Office which supports prime ministers and ensures the effective running of government, follows a monthslong probe into the "partygate" scandal.

A separate police investigation led to police issuing 126 fines to 83 people, including Johnson and Finance Minister Rishi Sunak. Johnson initially denied any lockdown rules had been broken in Downing Street when the allegations first emerged last year. He has since apologized for his fine and the various breaches but is refusing to heed calls from his political opponents and others to resign.

In her series of conclusions, Gray found "many of these gatherings and the way in which they developed was not in line with COVID-19 guidance at the time."

"I found that some staff had witnessed or been subjected to behaviours at work which they had felt concerned about but at times felt unable to raise properly," she concluded.

"I was made aware of multiple examples of a lack of respect and poor treatment of security and cleaning staff. This was unacceptable."

The top official said it was beyond her scope to recommend sanctions but reflected on the implications of the scandal.

"Many will be dismayed that behaviour of this kind took place on this scale at the heart of government," she noted.

"The public have a right to expect the very highest standards of behaviour in such places and clearly what happened fell well short of this."

The prime minister got a further reprieve when the Metropolitan Police told him last week that he would not be getting any more fines, but Gray's conclusions could revive calls from Conservative lawmakers for a no-confidence vote in the leader who won them a big parliamentary majority just over two years ago.