Boris Johnson will make “a big mistake” if he axes the post of an ethics adviser following Christopher Geidt’s sudden resignation, his former anti-corruption tsar is warning.

       No 10 has floated not replacing the peer – who quit in protest at being asked for advice on a “deliberate” breach of the ministerial code – despite the outcry over standards in government.

       But John Penrose, who also quit this month over the prime minister’s response to the Partygate scandal, warned he would pay a price if no-one scrutinises ministers’ behaviour.

       “That would be potentially quite a big mistake. You can obviously change the role a bit, but you shouldn’t be weakening the role,” he said.

       Warning Mr Johnson is already “overdrawn on his account” after repeated scandals, Mr Penrose said: “You can’t just pretend it doesn’t matter, that there’s no job to be done.”

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       He said: “They need to show that they’re serious about this”, adding: “The difficulty with issues about honesty and integrity, and so forth, is they don’t go away if you just ignore them.”

       Mr Penrose also questioned what would happen to Lord Geidt’s outstanding report into the controversy over the financing of the prime minister’s lavish flat refurbishment.

       He called for it to be completed and published, rather than be left “sitting on the shelf” after the departure of the adviser on ministerial interests.

       Meanwhile, the small business minister Paul Scully admitted to unanswered questions about Lord Geidt’s resignation – which No 10 claimed was about tariffs, apparently in the steel industry.

       Mr Scully said he “can’t really reconcile” Lord Geidt stating he had been placed in an “impossible and odious” position with the explanation for his departure.

       There is widespread scepticism that steel tariffs are a full explanation for the walkout – and bewilderment that the overseer of the ministerial code was asked for his advice in the first place.

       Lord Geidt is not believed to have been consulted when the government ignored its trade watchdog and extended the tariffs the first time a year ago.

       He has also not been consulted over far more high-profile issues – the ripping up of the Northern Ireland Protocol and the deportations to Rwanda – alleged to breach international agreements.

       This week, Lord Geidt hinted he would have investigated Mr Johnson – if he had been allowed to – over whether he breached the code in being fined over the No 10 parties.

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       He told MPs: “It’s reasonable to say that, perhaps a fixed penalty notice and the prime minister paying it, may have constituted not meeting the overarching duty under the ministerial code of complying with the law.”

       Catherine Haddon, senior fellow at the Institute for Government think tank, said Lord Geidt “was ready to walk and they gave him the bullet.”