Boris Johnson’s ethics adviser told the prime minister he was quitting after being placed in an “impossible and odious position”.
Downing Street published Lord Geidt’s resignation letter a day after he unexpectedly decided to step down, becoming the second in the role to do so under Mr Johnson.
Lord Geidt admitted this week that he had considered resigning over Mr Johnson’s response to his partygate fine for breaking COVID-19 rules.
But an exchange of letters with the PM explaining why he was leaving pointed to a separate issue, apparently relating to potential future decisions relating to trade tariffs.
In his letter to the prime minister explaining why he was stepping down, Lord Geidt said he was being asked to offer a view on “measures which risk a deliberate and purposeful breach of the ministerial code”.
He added: “This request has placed me in an impossible and odious position.
“My informal response on Monday was that you and any other minister should justify openly your position vis-a-vis the code in such circumstances.
“However, the idea that a prime minister might to any degree be in the business of deliberately breaching his own code is an affront.”
Downing Street had initially published only a very brief statement from Lord Geidt when it announced his resignation but came under pressure to reveal more.
His exit follows the resignation of his predecessor, Sir Alex Allan, in 2020 and the departure of the prime minister’s anti-corruption champion, John Penrose, last week.