British Prime Minister Boris Johnson's top ethics advisor said he had quit after the government forced him into an "impossible and odious" position, according to a letter released on Thursday.

Christopher Geidt is the second official to quit the role in two years in protest at Johnson's actions. His resignation on Wednesday came after the prime minister was enmeshed in the "Partygate" scandal, which saw him receive a police fine.

Lord Geidt had stayed on despite expressing unease over the Covid lockdown-breaching parties in Downing Street, and endured a bruising time in front of a committee of MPs on Monday.

At that hearing, Geidt had said it was "reasonable" to conclude that Johnson broke Britain's ministerial code by getting the police fine. Breaches of the code are traditionally a resigning matter.

But Johnson has refused to quit, and his office said it was "surprised" at the mandarin's decision to step down now, as the term "Geidtgate" began circulating on social media.

In his resignation letter to Johnson, Geidt said the final straw came when he was asked to advise on "a deliberate and purposeful breach of the ministerial code" by the government on an unspecified issue.

"This request has placed me in an impossible and odious position," he wrote.

"I can have no part in this," added Geidt, who had previously cleared Johnson over another scandal related to who paid for a lavish redecoration of his Downing Street flat.

Ministers refused to be drawn on the specifics of the row. But trade experts said it related to a ruling by other officials that the government could no longer justify its tariffs on Chinese steel.

No ethics left

In his letter of response, Johnson said he had sought Geidt's "advice on the national interest in protecting a crucial industry" through retaliatory trade tariffs.

The industry was not identified and Johnson said the government believed its proposed measure would comply with UK law.

But it "might be seen to conflict" with the UK's obligations under the World Trade Organization, the prime minister noted in seeking Geidt's counsel. 

Johnson last week narrowly survived a vote of no confidence by Conservative MPs in the House of Commons over the "Partygate" affair.

His party critics returned to the fray on Thursday after the exit of Geidt, who is a former private secretary to Queen Elizabeth II and was personally appointed by Johnson himself.

Tory MP William Wragg paraphrased the playwright Oscar Wilde as he ridiculed Johnson in parliament.

"For the prime minister to lose one adviser on ministers' interests may be regarded as misfortune. To lose two looks like carelessness," Wragg said.

Geidt's predecessor, Alex Allan, resigned in November 2020 after Johnson refused to fire interior minister Priti Patel, despite Allan finding that she had breached the ministerial code by bullying officials.

The position went vacant for five months before Geidt eventually took over. 

The prime minister will now review the role, and may leave it unfilled, his spokesman told reporters.

The opposition Labour party, which has been demanding Johnson's head for his "lies" over Partygate, was scathing.

"There are now no ethics left in this Downing Street regime propped up in office by a Conservative Party mired in sleaze and totally unable to tackle the cost-of-living crisis facing the British people," Labour's deputy leader Angela Rayner said.