Boris Johnson’s independent ethics adviser has refused to deny he considered resigning over the Prime Minister’s response to being fined for attending a party in Downing Street during lockdown.

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

However, he said that he did not believe there was ever a point when he formed “a single direct proposition” in his own mind.

The Times reported that Lord Geidt threatened to quit his post following the publication last month of the Sue Gray report into lockdown violations in Whitehall unless Mr Johnson issued a public explanation for his conduct.

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

He said he had acted in “good faith” when he told Parliament that there had not been any parties and that he had since corrected the record.

Asked about the report that he threatened to quit, Lord Geidt acknowledged that “the commentariat” had picked up on his “frustration” at that time.

“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.”

Pressed by Labour MP John McDonnell if he had contemplated resignation, Lord Geidt said: “There are few instruments available to an independent adviser and (it is) important to consider what is going to work best in the interest, not of me, but preserving the integrity of the system and of the (Ministerial) Code in making it work in advising the Prime Minister on holding ministers – including a prime minister – publicly to account.”

“I haven’t given you a direct answer but I don’t think there was ever a single, direct proposition in my own mind.”

Mr Mc Donnell replied: “I am going to take that answer as at least it was on the agenda.”

Lord Geidt said: “We have mentioned before in evidence that it is always on the agenda as an available remedy to a particular problem.”

Lord Geidt accepted that it was “reasonable” to suggest the Prime Minister may have breached the Ministerial Code as a result of having been issued with a fixed penalty notice by the Metropolitan Police.

“If I am to take the view of, say, the ordinary man or woman in the language as it were on the face of the code then I think … it’s reasonable to say that perhaps a fixed penalty notice and the PM paying for it may have constituted not meeting the overarching duty of the Ministerial Code of complying with the law,” he said.

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

He said that was an “asset of the Prime Minister as a minister of the Crown” rather than a “free-orbiting adviser” and still required Mr Johnson’s final consent before starting an investigation.

“The new terms of reference show there is still some small limitation on the capacity of the independent adviser to be truly independent,” he said.

Lord Geidt criticised the five-month delay in his appointment after his predecessor, Sir Alex Allan, quit in 2020 after Mr Johnson refused to accept his finding that Home Secretary Priti Patel had bullied civil servants.

“I will hold to the view that on balance it is right and proper that there should be an independent adviser in post to make sure that the regular business does not go unattended as was the case in that five-month lacuna,” he said.