Christopher Geidt, the second ethics adviser to quit under Johnson in two years, did not specify what advice the prime minister had requested but said it would have made a mockery of the so-called ministerial code, rules which set standards for government.

It is the latest resignation from Johnson’s government, which is under pressure on multiple fronts. The prime minister has been criticised for his role in the so-called “partygate” as well as his deportation policy to Rwanda, and the EU has said a new plan to deal with post-Brexit trade is illegal.

“This request has placed me in an impossible and odious position,” Geidt said in his resignation letter to Johnson, which was published by the government on Thursday.

“I can have no part in this,” said Geidt, who was an independent adviser on ministers’ interests.

Johnson, responding in a letter, said he was surprised by the resignation, adding the issue that Geidt was asked to consider was a potential decision about trade tariffs linked to a body called the Trade Remedies Authority.

He said the issue previously had enjoyed cross-party support but may have conflicted with Britain’s obligations under the World Trade Organization.

“My intention was to seek your advice on the national interest in protecting a crucial industry, which is protected in other European countries and would suffer material harm if we do not continue to apply such tariffs,” Johnson wrote.

“In seeking your advice before any decision was taken, I was looking to ensure that we acted properly with due regard to the ministerial code.”

Geidt seemed to suggest earlier this week that he was frustrated with the pressure he had been put under by the government in his role, and a spokesman for the prime minister said Johnson would reflect on how best to fill the role.

Citing commercial sensitivity for not giving more details about the advice that was sought, the spokesman said it was a “relatively unusual situation” regarding an industry.

British trade experts said it was most probably related to steel tariffs.

The previous holder of the ethics role resigned in 2020 in protest of Johnson’s support for a minister who was found to have broken the code.

The government initially only published a brief statement from Geidt when it announced his resignation on Wednesday, but came under pressure to reveal the reasons for his departure.