In a second letter outlining why he had quit, Lord Geidt said reports it was related to steel tariffs were a “distraction”.
“I could not be a party to advising on any potential law-breaking,” he said.
Lord Geidt was the second ethics adviser to quit under Mr Johnson, after Sir Alex Allan left in 2020.
He left his role on Wednesday, accusing the prime minister of putting him in an “impossible and odious position” over an issue he believed would amount to a breach of the ministerial code – the rules about standards in government.
In a letter seen by BBC News on Friday evening, he elaborated on why he quit.
Lord Geidt told the chairman of the Public Administration and Constitutional Affairs Committee, Conservative MP William Wragg, there had been “confusion about the precise cause of my decision”.
He said his concern expressed in his letter to the PM was “simply one example of what might yet constitute deliberate breaches by the United Kingdom of its obligations under international law”.
In his letter, Lord Geidt added that former Cabinet Secretary Lord Butler had represented his position “precisely” when he said, in a BBC interview on Thursday, that he had believed he should not “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”.
“My letter has been interpreted to suggest that an important issue of principle was limited to some narrow and technical consideration of steel tariffs,” Lord Geidt wrote.
“The cautious language of my letter may have failed adequately to explain the far wider scope of my objection.”
In his original resignation letter, Lord Geidt said he had come close to quitting over breaches of Covid lockdown rules in Downing Street.
But he said he ultimately stood down after had been asked for advice this week on another issue he believed would amount to a deliberate breach of the code.
Lord Geidt “wouldn’t have taken this decision lightly” – former PM adviser
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 on Friday it was right for the PM to take time to reflect on whether Lord Geidt should be replaced.
A Number 10 spokesman said: “It may be that the prime minister decides to make a like-for-like replacement, or it might be that we set up a different body that undertakes the same functions.”