Boris Johnson’s former ethics adviser has said the reason given by Downing Street for his resignation was a “distraction” and doubled down on claims that the government wanted to break international law.

After he dramatically quit this week, Christopher Geidt said his explanation had used too much “cautious language” leading to “some confusion about the precise cause of my decision”.

In Lord Geidt’s initial letter to No 10 on Wednesday, he said he had been asked to give advice on an “odious” breach of the ministerial code. The response from Johnson suggested this was over a plan to extend steel tariffs in contravention of World Trade Organization rules.

However, after much mystery over why Geidt decided to quit over this issue and not greater concerns around Partygate, he issued a “clarification about the reasons for my departure”. “There has been some confusion about the precise cause of my decision,” he wrote in a letter to the Tory MP William Wragg, the chair of the public administration and constitutional affairs committee (PACAC).

“My letter has been interpreted to suggest that an important issue of principle was limited to some narrow and technical consideration of steel tariffs. The cautious language of my letter may have failed adequately to explain the far wider scope of my objection.”

Geidt, a former private secretary to the Queen, said the emphasis on the steel tariffs issue was a distraction and “simply one example of what might yet constitute deliberate breaches by the United Kingdom of its obligations under international law, given the government’s widely publicised openness to this”.

While the explicit reference to international law was removed from the ministerial code in 2015, Geidt said there was “no explicit derogation, no let-off written into the code to absolve individual ministers of their own obligations”. He said given his commitment to integrity, “I could not be a party to advising on any potential law-breaking”.

Angela Rayner, Labour’s deputy leader, said Geidt had quit “because of the odious behaviour of Boris Johnson’s Downing Street”, and added: “It’s high time for Tory MPs to do the decent thing by showing this rotten, rule-breaking prime minister the door.”

Karin Smyth, a Labour MP who sits on PACAC, said the Geidt letter had provided “helpful clarity”, but that “it isn’t steel that broke the camel’s back”.

The government was accused earlier in the week of threatening to breach international law by publishing plans to unilaterally override the Northern Ireland protocol signed by Johnson as part of his Brexit deal.

Westminster insiders speculated that Geidt could have been referring to the issue without explicitly referencing it in his latest letter when he referred to concerns about ministers breaking international law.

Geidt is the second ethics adviser to quit under Johnson. In November 2020 Alex Allen stepped down after his finding that the home secretary, Priti Patel, had breached the ministerial code by bullying staff was brushed aside by No 10.

Downing Street has launched a review of the ethics adviser role and has not confirmed whether it will replace Geidt.