British Prime Minister Boris Johnson's independent ethics adviser has resigned from his post as Independent Adviser on Ministers' Interests in the wake of the partygate scandal of Covid lockdown-breaching parties at Downing Street.

Lord Christopher Geidt, the Independent Adviser on Ministers' Interests who reported directly to the Prime Minister in an advisory capacity, handed in his resignation to Johnson on Wednesday evening. His resignation statement does not specify a reason for his departure and Downing Street is yet to elaborate on that.

"With regret, I feel that it is right that I am resigning from my post as Independent Adviser on Ministers' Interests," Geidt said in a brief statement.

The resignation has taken Downing Street by surprise and follows the 2020 exit of Sir Alex Allan, his predecessor in the role, and the resignation of the British Prime Minister's anti-corruption champion, John Penrose, last week.

"Whilst we are disappointed, we thank Lord Geidt for his public service," a government spokesperson said.

According to reports, Geidt had been asked to provide advice on a commercially sensitive matter in the national interest , which previously had cross-party support.

No decision had been taken pending that advice," the government spokesperson added.

Deputy Prime Minister Dominic Raab said he was unaware of the details behind the resignation and that Downing Street would provide an update later.

Meanwhile, the Opposition parties renewed their partygate attack on the government over this latest development.

"The Prime Minister has now driven both of his own handpicked ethics advisers to resign in despair. If even they can't defend his conduct in office, how can anyone believe he is fit to govern," said Labour's Deputy Leader Angela Rayner.

"When both of Boris Johnson's own ethics advisers have quit, it is obvious that he is the one who needs to go," questioned Liberal Democrat chief whip Wendy Chamberlain.

Last month, in an annual report on ministerial interests, Geidt had noted 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.

His report called on Johnson to 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 was 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 a public comment on his obligations under his own Ministerial Code, a breach of which usually leads to a minister's resignation.

It had been reported at the time that Geidt had threatened to quit after the publication of the Sue Gray report into lockdown breaches in Downing Street unless Johnson issued a public explanation for his conduct.