Boris Johnson’s ethics adviser has resigned a day after telling MPs it was “reasonable” to suggest the Prime Minister broke the ministerial code by breaching lockdown laws.

Lord Geidt became the second ministerial interests adviser to resign during the Prime Minister’s three years in office when a brief statement was published 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 message on the Government website read.

No reason was given for his shock departure, and a senior source in No 10 told the PA news agency Mr Johnson was “surprised” by Lord Geidt’s resignation, adding: “This is a mystery to the PM.”

Labour renewed its calls for Mr Johnson to resign, accusing him of having “driven both of his own hand-picked ethics advisers to resign in despair”.

A little over 24 hours earlier, the crossbench peer declined to deny that he had previously considered quitting over Mr Johnson’s response to being fined by police over partygate.

Lord Geidt told the Commons Public Administration and Constitutional Affairs Committee that he had felt “frustration” and that the option of resignation was always “on the agenda”.

“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,” he said.

But after Lord Geidt took the move he once described as a “last resort”, William Wragg, the Conservative MP who chairs the committee, praised him as “a person of great integrity, motivated by the highest ideals of public service”.

“For the Prime Minister to lose one adviser on ministers’ interests may be regarded as a misfortune. To lose two looks like carelessness,” the critic of Mr Johnson said.

Downing Street hinted towards a recent request for Lord Geidt to advise on an undisclosed “commercially sensitive matter” as being behind the resignation.

“This week, the independent adviser was asked to provide advice on a commercially sensitive matter in the national interest, which has previously had cross-party support. No decision had been taken pending that advice,” a Government spokeswoman said.

“Whilst we are disappointed, we thank Lord Geidt for his public service. We will appoint a new adviser in due course.”

It was reported that Lord Geidt had threatened to quit last month after the publication of the Sue Gray report into lockdown breaches in Whitehall unless Mr Johnson issued a public explanation for his conduct.

In response, the Prime Minister issued a letter to Lord Geidt saying he believed any breach of Covid laws when he attended a gathering in the Cabinet room for his 56th birthday had been “unwitting”.

He insisted he was acting in “good faith” when he told Parliament there had not been any parties.

On Tuesday, Lord Geidt, a former private secretary to the Queen, accepted it was “reasonable” to suggest the Prime Minister may have breached the ministerial code by being handed a fixed-penalty notice by the Metropolitan Police.

However, he indicated to MPs he would not be launching an investigation into Mr Johnson even though he has since gained greater powers to initiate his own inquiries.

He described himself as an “asset of the Prime Minister” rather than a “free-orbiting adviser”, and still required Mr Johnson’s final consent before starting an investigation.

Deputy Labour leader Angela Rayner said: “The Prime Minister has now driven both of his own hand-picked 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?

“The person who should be leaving No 10 tonight is Boris Johnson himself. Just how long does the country have to wait before Tory MPs finally do the right thing?”

Chris Bryant, the Labour MP who chairs the Commons Standards Committee, praised Lord Geidt as “one of the most honourable men I have ever met”.

“He thought he could discreetly bring about incremental change but he was repeatedly lied to by No 10. In honour Johnson should resign,” Mr Bryant added.

The No 10 source suggested Lord Geidt had indicated he would like to stay in the role until the end of the year at least.

“We are surprised, this is a mystery to the PM. Only on Monday he said he would like to stay on for another six months,” the source said.

The first of Mr Johnson’s ethics advisers to quit was Sir Alex Allan, who resigned in 2020 after the Prime Minister refused to accept his finding that Home Secretary Priti Patel had bullied civil servants.

Lord Geidt lasted 14 months in the role, a period he described as “especially busy”.

He accepted the role in April last year, with the post having been vacant for five months after Sir Alex’s resignation.

After a month in the job, Lord Geidt cleared Mr Johnson of breaching the rules over the lavish refurbishment of his Downing Street flat, after payments were made by Tory donor Lord Brownlow.

But the adviser said Mr Johnson had acted “unwisely” by allowing the work to go ahead without “more rigorous regard” for how it was being funded.

In January the Prime Minister was forced to offer a “humble and sincere apology” to Lord Geidt for not disclosing crucial messages with Lord Brownlow.

Lord Geidt expressed his “grave concern” over the incident that “shook his confidence” after the exchanges emerged in an Electoral Commission investigation.