Dominic Raab has said he does not know why Boris Johnson’s ethics adviser quit – as Number 10 came under pressure to explain what was behind the decision.

Mr Raab batted off the suggestion that the departure of Lord Geidt reflected badly on the prime minister in an interview with Sky News – and said Downing Street would provide an update later.

The deputy prime minister pointed instead to the “grilling” that Lord Geidt had in front of a select committee this week and advice he had been asked for on a “commercially sensitive” matter as potential factors.

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Lord Geidt’s decision is said to have taken Downing Street by surprise and follows the exit of Sir Alex Allan, his predecessor in the role, in 2020, and the resignation of the prime minister’s anti-corruption champion, John Penrose, last week.

Last month, Lord Geidt had said there was a “legitimate question” over whether the prime minister broke the ministerial code after he received a partygate fine for going to a birthday party in the Cabinet Room in June 2020 when indoor socialising was banned.

On Tuesday, he refused to deny to MPs that he had considered resigning over Mr Johnson’s response to the fine.

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Asked about whether Lord Geidt’s departure, on top of those of Sir Alex and Mr Penrose, reflected badly on Mr Johnson, Mr Raab said: “It’s not clear to me at all what the reason is.

“There are clearly these issues around the commercially sensitive matter which he was engaged on, or asked to advise on.

“My understanding was that, as of earlier this week, he was hoping to stay on for another six months.

“He’s obviously had that grilling in front of MPs. So the risk is you’re adding one plus one and getting three.

“There will be a further update from Number 10 later today.”

Asked whether he believed the government always acted ethically, Mr Raab said: “Yes, I do. I think we are doing our best for the country.

“Do we make mistakes? Yes, it happens – we’re human, we’re fallible.”

No resignation letter has been published since Lord Geidt’s departure was announced on Wednesday evening.

A brief statement read: “With regret, I feel that it is right that I am resigning from my post as Independent Adviser on Ministers’ Interests.”

When asked why the government has not published Lord Geidt’s resignation letter, Mr Raab said: “I don’t know whether there’s a full letter”. He said the adviser will be replaced “as soon as possible”.

Labour’s shadow international trade secretary Nick Thomas-Symonds told Sky News: “I don’t think it is surprising, given the appalling conduct of this prime minister over a sustained period of time that we see, again, an independent ethics adviser leaving.

“What we can’t have here is Number 10 somehow not making public the reason… we need to see a situation where Number 10 is open about this.”

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