Further details of Lord Geidt’s shock resignation as Boris Johnson’s ethics adviser will be set out on Thursday.

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

Ministers said the Government was “disappointed” that Lord Geidt had quit and both his resignation letter to Mr Johnson and the Prime Minister’s reply will be released.

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, 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.

Deputy Prime Minister Dominic 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.”

Cabinet Office minister Michael Ellis, responding to an urgent question from Labour, told MPs in the Commons: “The Prime Minister will be issuing a letter in relation to Lord Geidt’s announcement. Both Lord Geidt’s letter and the Prime Minister’s reply will be deposited in the House shortly.”

He added: “The Government is particularly disappointed, of course, that Lord Geidt has taken this decision as only very recently, as this House knows from the debate last week, significant changes were made to the role and status of the independent adviser on ministers’ interests.

“These changes represent the most substantial strengthening of the role of the independent adviser since its creation.”

Mr Ellis refused to give further details about the “sensitive matter” Lord Geidt had been asked to advise on.

Labour MP Fleur Anderson said: “Can the minister confirm whether this relates to a direct or indirect financial interest of the Prime Minister, a family member, a friend or a donor?”

But Mr Ellis said on “sensitive matters” it was ” not obviously appropriate to dwell on those, as is clear”.

“The letters will speak for themselves,” he added.