The resignation comes amid controversy over whether the PM broke the ministerial code.
Boris Johnson’s ethics adviser Lord Geidt has resigned after saying there was a “legitimate question” over whether the PM had broken ministerial rules over Partygate.
He said he was leaving the role, which he took up in April last year, “with regret”.
Lord Geidt did not give a reason for his departure but in a statement he said it was the “right thing” to do.
It comes a day after he spoke of “frustrations” with the PM’s actions.
Lord Geidt’s predecessor as ethics adviser, Sir Alex Allan, resigned after Mr Johnson overruled him over a report into alleged bullying by Home Secretary Priti Patel.
Labour deputy leader Angela Rayner said: “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?”
But a Downing Street source told the BBC: “This is a total surprise and a mystery to the PM. Only on Monday Lord Geidt asked if he could stay on for six months.”
It was reported that Lord Geidt had threatened to quit last month after the publication of the Sue Gray report into lockdown breaches in Downing Street unless Mr Johnson issued a public explanation for his conduct.
Appearing before a committee of MPs on Tuesday, Lord Geidt said: “Resignation is one of the rather blunt but few tools available to the adviser. I am glad that my frustrations were addressed in the way that they were.”
In a brief written statement on Wednesday, he said: “With regret, I feel that it is right that I am resigning from my post as independent adviser on ministers’ interests.”
Lord Geidt’s complaints about his role haven’t been made forcefully or angrily but his discomfort has been obvious.
He suggested to MPs that he hadn’t been granted enough power to be truly independent and admitted his advice had been ignored by the prime minister.
This week Downing Street had been celebrating a return to policy debate rather than Partygate, but Lord Geidt has put the spotlight back on Boris Johnson’s behaviour.
The PM’s critics say he’s never believed rules apply to him.
Losing his second adviser on ethics will add to that impression.
Mr Johnson was fined in April over a surprise birthday party in his honour that he attended in Downing Street in June 2020.
Writing to Lord Geidt afterwards, he said there had been “no intent” to break Covid regulations”, and that he had been “fully accountable to Parliament and the British people”.
The ministerial code, which outlines the rules government ministers must follow, says there is an “overarching duty” on them to comply with the law.
If the code is broken, the convention in Westminster is for a minister to resign.
In his annual report on ministers’ interests, published on 1 June, Lord Geidt said questions around Mr Johnson’s behaviour had led to an “impression… the prime minister may be unwilling to have his own conduct judged against” the ministerial code.
He said that, when it came to the Partygate fine, “a legitimate question has arisen as to whether those facts alone might have constituted a breach of the overarching duty within the ministerial code of complying with the law”.
Liberal Democrat chief whip Wendy Chamberlain said: “When both of Boris Johnson’s own ethics advisers have quit, it is obvious that he is the one who needs to go.”
Lord Geidt previously served as the Queen’s private secretary and before that he was an Army intelligence officer and diplomat.