Boris Johnson’s ethics adviser has said it was “reasonable” to suggest the prime minister may have breached the ministerial code when he was fined during the Partygate scandal.

Giving evidence to MPs, Christopher Geidt suggested he did not have the power to investigate Johnson’s potential breach linked to lockdown parties and that he had not requested an investigation, but instead had required a statement from the prime minister – who cleared himself of any breach.

Lord Geidt was reported to be on resignation watch after Johnson wrote to him clearing himself of breaching the ministerial code over Partygate, after the adviser said there was a “legitimate question” about whether he had done so.

Amid controversy over Johnson’s changes to the ministerial code, Geidt told the committee he now had enhanced powers – which he believed gave him the power to initiate investigations into the prime minister.

“I’m glad to get the new power and I’m not going to be restrained from using it where necessary but my powers were less clear in the previous period,” he said.

Giving evidence to MPs on Tuesday, Geidt appeared to cast doubt on the independence of his role because of the necessity of reporting directly to the prime minister. He said there was a “standing question” about the constitutional basis of the role, which he said was “assisting the prime minister in the business of managing his own ministers”.

“The point there is I’m an asset of the prime minister as a minister of the crown, rather than a free-orbiting adviser with a different source of authority,” he told the public administration and constitutional affairs committee. “There is some small limitation on the capacity of the independent adviser to be truly independent.”

Geidt, who has been in office through multiple public scandals, including the Wallpapergate saga over donations to Johnson’s Downing Street flat, as well as breaches of lockdown rules through parties in Downing Street, drew laughter from committee members when he said it had been an “exceptionally busy year”.

He said the “ordinary man or woman” might have concluded that it was “reasonable to say that perhaps a fixed-penalty notice and the prime minister paying for it may have constituted not meeting the overarching duty of the ministerial code of complying with the law”.

Asked on Tuesday if he had considered resigning after the response was published, he said he did not believe there was ever “a single direct proposition” in his own mind.

“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 said. “Resignation is one of the blunt but few tools available to the adviser. I am glad that my frustrations were addressed in the way that they were.”

He said he had “never expected it to be easy” in the role, but added: “How can I defeat the impression of a sort of cosy … relationship, I think it’s very hard, I will freely admit, but I’m trying my best to work with what I’ve got. And the changes that have been recorded I think are useful, they’re workable.

“Yes, they’re incremental. But insofar as this has always been a prime ministerial appointment, because of its service to the crown’s chief minister, I have tried in my short time to discharge it as well as I can. Notwithstanding that it’s been done in a slightly brighter glare of publicity than I think is usually comfortable.”