Boris Johnson's former ethics chief Lord Geidt said part of the reason for his resignation was that he could not be "party to advising on potential law breaking".
In Geidt’s initial letter to No 10 on Wednesday, he said the PM forced him into an “impossible and odious” position, with a request for advice which he thought would have amounted to a breach of the ministerial code.
The response set out from the prime minister indicated that this was over a plan to extend steel tariffs, which would have potentially broke World Trade Organization (WTO) rules.
On Friday, after fevered speculation over other motives, he sought to clarify his position in a letter to Constitutional Affairs Select Committee Chair William Wragg.
"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 cross bench peer wrote.
"The cautious language of my letter may have failed adequately to explain the far wider scope of my objection."
Lord Geidt went on to write that the former Cabinet Secretary Lord Butler summed up his motivation for resigning accurately in a broadcast interview he gave on Thursday.
‘He [Lord Geidt] thought it odious and impossible that he should be asked to give cover on something that might be in breach of international law and he didn’t think that that was something that ought to be asked of him.
"This isn’t about steel. It’s about whether Lord Geidt should be asked to give advanced cover to the prime minister where there is contemplation of doing something that may be in breach of international law."
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Lord Geidt said he had been narrowly clinging on as ministerial interests adviser over partygate but ultimately quit after being forced into an “impossible and odious” position by the prime minister.
In his letter to Mr Wragg, he suggested that he could not be connected to any potential breach of international law.
"Although explicit reference to international law was removed from the Ministerial Code in 2015, it is widely still held that a breach of international law would, in turn, represent a prima facie breach of the ministerial code.
"I could not be a party to advising on any potential law-breaking."
Downing Street responded to the resignation on Wednesday night, saying in a statement "we are surprised by this decision" and adding the government was "disappointed".
His exit came after he told MPs it was “reasonable” to suggest Mr Johnson broke the code by being fined by police for breaching Covid laws. In his letter, he said he was “disappointed” that the prime minister did not give a fuller account over how paying the fixed penalty notice did not breach the code.
Lord Geidt's predecessor Sir Alex Allan quit in 2020 after Mr Johnson refused to accept his finding that Home Secretary Priti Patel had bullied civil servants.
He began the job last April by leading an inquiry into the funding of renovations to Mr Johnson’s Downing Street flat following reports over the cost of the work.