• He said the idea that the prime minister "might to any degree be in the business of deliberately breaching his own code is an affront" that would suspend the code "to suit a political end"
• In his response, Mr Johnson said his intention was to seek Lord Geidt's "advice on the national interest in protecting a crucial industry".
Boris Johnson's ethics adviser says he quit after the PM forced him into an “impossible and odious” position.
Lord Geidt's job was to advise the PM on the ministerial code - a set of rules governing standards of behaviour.
In his resignation letter, he said he had come close to quitting over Covid law-breaking in Downing Street.
But he said he had been asked for advice this week on another issue he believed would amount to a deliberate breach of the code.
"This request has placed me in an impossible and odious position," wrote Lord Geidt in his letter.
He said the idea that the prime minister "might to any degree be in the business of deliberately breaching his own code is an affront" that would suspend the code "to suit a political end".
"This would make a mockery not only of respect for the code but licence the suspension of its provisions in governing the conduct of Her Majesty's ministers," Lord Geidt wrote.
"I can have no part in this."
In his response, Mr Johnson said his intention was to seek Lord Geidt's "advice on the national interest in protecting a crucial industry".
Downing Street said Mr Johnson was reviewing whether or not to fill the vacant position, and declined to comment on suggestions the plan related to maintaining tariffs on Chinese steel despite possibly breaching World Trade Organisation (WTO) commitments.
Image: SUE GRAY REPORT
Lord Geidt is the second ethics adviser to quit under Mr Johnson's premiership.
Sir Alex Allan - who resigned from the role in 2020 - said it was "dreadful that an honourable man like Lord Geidt has been placed in a position where he felt he had no option but to resign."
Labour's shadow attorney general Emily Thornberry said the two resignations showed there was "something really rotten at the heart of Downing Street".
She told reporters Lord Geidt was a "man at the end of his tether" who had realised it's "not possible" to defend the prime minister and had "had enough".
Liberal Democrat leader Sir Ed Davey said Parliament should be allowed to vet the next ethics adviser "so that Johnson can't simply appoint one of his cronies".
The SNP MP Brendan O'Hara said the resignation was "another humiliation for the prime minister".
In his resignation letter, Lord Geidt expresses concern about the prime minister's reaction to widespread Covid rule-breaking in Downing Street, as highlighted in Sue Gray's report.
He says he had decided it was possible to "continue credibly" as the PM's adviser "albeit by a very small margin".
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Lord Geidt is the second ethics adviser to quit under Mr Johnson's premiership.
Sir Alex Allan - who resigned from the role in 2020 - said it was "dreadful that an honourable man like Lord Geidt has been placed in a position where he felt he had no option but to resign."
Labour's shadow attorney general Emily Thornberry said the two resignations showed there was "something really rotten at the heart of Downing Street".
She told reporters Lord Geidt was a "man at the end of his tether" who had realised it's "not possible" to defend the prime minister and had "had enough".
Liberal Democrat leader Sir Ed Davey said Parliament should be allowed to vet the next ethics adviser "so that Johnson can't simply appoint one of his cronies".
The SNP MP Brendan O'Hara said the resignation was "another humiliation for the prime minister".
In his resignation letter, Lord Geidt expresses concern about the prime minister's reaction to widespread Covid rule-breaking in Downing Street, as highlighted in Sue Gray's report.
He says he had decided it was possible to "continue credibly" as the PM's adviser "albeit by a very small margin".