UK Prime Minister Boris Johnson braced on Wednesday for the release of a senior civil servant's long-awaited full report into a "Partygate" scandal, as new allegations emerged about a culture of lockdown-breaking boozing in his offices.

A photograph published by the Daily Mirror newspaper showed a Downing Street table laden with wine bottles and doughnuts, and it said an accompanying WhatsApp message told staff: "Time to open the Covid secure bar."

But the Mirror said that particular event in November 2020 was thought not to have been investigated by civil servant Sue Gray or London's Metropolitan Police, which has issued multiple fines over other events, including one against Johnson himself.

The prime minister has defied calls to resign after he received the fine, but multiple reports said MPs from his Conservative party were waiting to see the full details revealed by Gray before deciding whether to trigger a leadership ballot.

The report was expected on Wednesday, once it is cleared for release by Johnson, according to sources.

Gray released a preliminary version of her report in January, but held off fuller publication as the Met announced its own investigation.

That is now complete with the issuance of 126 fines to 83 people, although the police force is under pressure to reopen the investigation as new evidence emerges.

The BBC's Panorama programme on Tuesday interviewed people who attended another leaving party in November 2020, describing a rule-breaking culture with dozens of people crowded into the room.

The party came days after the government ordered a second Covid lockdown in England and banned households from mixing. (AFP)