The shock resignation of Boris Johnson’s ethics adviser may have been motivated by the grilling he received from MPs, Deputy Prime Minister Dominic Raab said.

Lord Geidt quit after he said it is “reasonable” to suggest the Prime Minister broke the ministerial code over coronavirus lockdown-busting parties in No 10.

He 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, though the reasons for his departure remain a mystery.

“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.

Downing Street issued a statement referring to Lord Geidt giving advice on a “commercially sensitive matter in the national interest” this week, but would not give further details.

Mr Raab, who is also the Justice Secretary, suggested a “pretty rough” appearance before the Commons Public Administration and Constitutional Affairs Committee on Tuesday could have contributed to Lord Geidt’s departure.

He said Lord Geidt had been in talks with Mr Johnson this week about staying on in the role for six months and appeared “committed” to the job.

But Mr Raab told Sky News: “I think he had a pretty rough grilling by MPs this week. I think sometimes we in the media and as politicians maybe underestimate how civil servants feel with that kind of scrutiny.”

He added “there was a particular issue, a commercially sensitive matter in the national interest, which he was asked to look at” – although Mr Raab did not give further details.

Downing Street is under pressure to release Lord Geidt’s formal resignation letter in the hope it could shed more light on his exit.

Mr Raab said there would be a “proper update” from No 10 later on Thursday.

Sir Philip Mawer, who served as Gordon Brown’s adviser on ministerial standards, told BBC Radio 4’s Today: “All I can say is that, if the letter and the Prime Minister’s reply are not published, then I think people will draw their own conclusions and it won’t be favourable to the Prime Minister.”

Labour’s Chris Bryant, chairman of the Commons Standards Committee, told Today: “Reading between the lines and between all the various different reports he has produced, he (Lord Geidt) basically thinks that the Prime Minister has broken the ministerial code himself.”

Calling for the resignation letter to be published, he said: “Downing Street does this all the time. It says there is no more to be seen and then subsequently we discover there was an incriminating letter.”

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.”

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.

Labour’s deputy leader Angela Rayner accused Mr Johnson of driving “both of his own hand-picked ethics advisers to resign in despair”.

She said: “If even they can’t defend his conduct in office, how can anyone believe he is fit to govern?”

Former Cabinet secretary Lord Turnbull, a crossbench peer, said Mr Johnson is “not worthy” of office and suggested Tories should work to overthrow him.

The peer, who was the most senior civil servant between 2002-2005, told BBC Newsnight: “It’s going to be solved when enough of his backbenchers can summon up the courage to decide that he’s not a man of sufficient integrity that they want as their leader.”

A little over 24 hours before his exit, 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”.

William Wragg, the Conservative MP who chairs the committee, said: “For the Prime Minister to lose one adviser on ministers’ interests may be regarded as a misfortune. To lose two looks like carelessness.”

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.