Lord Geidt has resigned as the prime minister’s adviser on ministerial interests.

He tendered his resignation to Boris Johnson, according to a brief statement on the government’s website on Wednesday evening.

“With regret, I feel that it is right that I am resigning from my post as Independent Adviser on Ministers’ Interests,” the statement reads.

A senior source in Number 10 said Lord Geidt’s resignation came as “a total surprise” and is “a mystery” to Mr Johnson.

“Only on Monday Lord Geidt asked if he could stay on for six months,” the source added.

Last month, Lord Geidt had said there was a “legitimate question” over whether the PM broke the ministerial code after he received a partygate fine over a birthday party in the Cabinet Room in June 2020 when indoor socialising was banned.

In his annual report, the ethics adviser said: “In the case of the fixed penalty notice recently issued to and paid by the Prime Minister, 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.”

On Tuesday, Lord Geidt refused to deny to MPs that he had considered resigning over Mr Johnson’s response to being penalised for breaching lockdown rules amid the scandal.

Lord Geidt told the Public Administration and Constitutional Affairs Committee that he had felt “frustration” over the lockdown-busting events that took place in Downing Street and across Whitehall.

“I am glad that the prime minister was able to respond to my report and in doing so addressed aspects of the things about which I was clearly frustrated,” he told the committee.

“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.”

Mr Johnson has repeatedly maintained that he did not break the ministerial code.

“In relation to the fixed penalty notice for my attendance in the Cabinet Room on June 19 2020, I believe that, taking account of all the circumstances, I did not breach the code,” he wrote in a responding letter to Lord Geidt in May.

It was previously reported that Lord Geidt had threatened to quit after the publication of the Sue Gray report if the PM did not publicly explain his actions.

Lord Geidt is the second person to resign as Mr Johnson’s ethics adviser during his less than three years as prime minister.

Sir Alex Allan quit in 2020 after Mr Johnson refused to accept his finding that Home Secretary Priti Patel had bullied civil servants.

Labour’s deputy leader Angela Rayner questioned how anyone can believe the PM “is fit to govern”.

“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?” she said in a statement.

“Yet he remains propped up in office by a Conservative party that is mired in sleaze and totally unable to tackle the cost of living crisis facing the British people.

“The person who should be leaving Number 10 tonight is Boris Johnson himself. Just how long does the country have to wait before Tory MPs finally do the right thing?”

Labour’s Chris Bryant, chair of the Committees on Standards and Privileges, added that it should be the PM who resigns and not Lord Geidt.

“Christopher Geidt is one of the most honourable men I have ever met,” he posted on social media.

“In the end, he was a decent man working for an indecent prime minister.

“He thought he could discreetly bring about incremental change but he was repeatedly lied to by Number 10.

“In honour, Johnson should resign.”

Legal activist Gina Miller was one of the first to react to Lord Geidt’s resignation on social media, tweeting: “Finally!”

Earlier this year, the PM was criticised by Lord Geidt over an official inquiry into the refurbishment of his Downing Street flat for failing to disclose messages he had exchanged with a Tory peer who initially paid for the renovations.

Lord Geidt said it demonstrated “insufficient” respect for his role.

Mr Johnson offered a “humble and sincere apology” to Lord Geidt over the matter.

The year prior, Lord Geidt had found that the PM did not breach the ministerial code over the refurbishment.