British Prime Minister Boris Johnson has been forced to explain, by his own independent ethics adviser, why he believed that the partygate fine slapped on him by Scotland Yard did not breach the country's Ministerial Code.

Lord Christopher Geidt, who reports directly to the Prime Minister in an advisory capacity, said in an annual report on ministerial interests released on Tuesday that a legitimate question has arisen as to the fixed penalty notice issued by the Metropolitan Police over a COVID lockdown-breaching birthday party at Downing Street in June 2020.

The report notes that Johnson must set out his case in public.

"I did not consider that the circumstances in which I received a fixed-penalty notice were contrary to the regulations," Johnson said in a letter of explanation to Geidt, which has been made public.

"I have accepted the outcome and paid it in compliance with legal requirements. Paying a fixed-penalty notice is not a criminal conviction," he said.

Geidt was also critical in his report about having repeatedly counselled the Prime Minister's advisers that he must offer public comment on his obligations under his own Ministerial Code, a breach of which usually leads to a minister's resignation.

That advice has not been heeded and, in relation to the allegations about unlawful gatherings in the Downing Street, the Prime Minister has made not a single public reference to the Ministerial Code, said the ethics adviser, who, according to "The Times", was on the verge of resigning over the issue.

While Johnson has vehemently defended his actions and blamed the delay in his addressing the Ministerial Code on a failure of communication between offices, it marks yet another push towards a slow but steady build-up of discontent over his leadership.

Several of his own Conservative Party members of parliament have been openly critical of his actions over partygate and many have also called for him to step down as leader and the Prime Minister.

For Tory MPs to topple their leader, 15 per cent of them have to write to the chair of the party's powerful 1922 Committee of backbench MPs. The current number stands at 54 MPs and only the chair, Sir Graham Brady, is aware of the exact number of letters before revealing that the threshold has been met to trigger a vote of no confidence.

If Johnson, 57, lost a confidence vote, he would be replaced as Conservative leader and prime minister. If he won, he could not face another challenge for a year.

So far, that number does seem far from hitting the target, but the fresh rows are making it difficult for Johnson to move on from the scandal, as he had hoped after delivering another apology in parliament following the scathing report by top civil servant Sue Gray last month.

Johnson repeated this in his letter to Geidt, saying he had taken full responsibility for everything that took place on my watch, and reiterate my apology to the House and to the whole country .

Johnson, as well as his wife Carrie and Chancellor Rishi Sunak, were fined by Met police for breaking Covid laws by attending one of the gatherings investigated by Gray.

The Opposition Labour Party's deputy leader, Angela Rayner, said Geidt's report was "the latest sign of the rampant sleaze engulfing Downing Street", adding: "This Prime Minister has been found out and his days are numbered."

It comes as Rayner and Labour Leader Keir Starmer received police questionnaires as part of an inquiry into alleged COVID rule-breaking at an election-related gathering in Durham in April last year, at which Starmer was pictured with a beer -- resulting in that being referred to as beergate .

Both leaders have denied any breach of rules at the work-related event and pledged to resign if issued with fines.

If the police decide to issue me with a fixed penalty notice, I would of course do the right thing and step down. People are entitled to expect that politicians followed the same rules as everyone else, Starmer has said, amid calls for Johnson to resign over his partygate fine.

During the coronavirus lockdown rules at the time, there was an exemption for work purposes and also if a gathering is reasonably necessary for the purposes of campaigning in an election.

The local Durham Constabulary initially decided that no offence had occurred but the force went on to announce an investigation saying it had since received "significant new information".