John Penrose has warned No 10 against refusing to appoint a new ethics adviser.
Lord Geidt tendered his resignation from the role on Wednesday evening, citing concerns over potential plans to breach WTO rules.
It follows months of speculation that Geidt could quit over the Partygate scandal.
Penrose, who resigned as the government’s anti-corruption tsar earlier this month argued that it would be a “big mistake” to scrap the role.
The Weston-super-Mare MP told the BBC: “Well, I think that that would be potentially quite a big mistake. The point here is that you can obviously, and if they felt it was sensibly it might be useful to look at it, you can obviously change the role a bit but you shouldn’t be weakening the role if you are going to come up with a revised version as a successor to Lord Geidt, some new format, some new way of dealing with the issue.”
He argued that the “parameters” of the independent adviser on ministers’ interests job could be adapted but that Geidt’s former role should continue to exist.
“So by all means change the parameters but I don’t think you can weaken it, particularly at the moment.
“I just think the prime minister is currently overdrawn, if I can put it that way, on his account with both the voters and with the Parliamentary party.
“They need to show that they are serious about this. This is part of the reset, I would argue, which the prime minister has rightly said he wants to do after last week’s vote of no confidence, good for him. This would be a good way of being part of that then moving it forward.”
Business minister Paul Scully was quizzed by Sky News over whether a new appointment would be made, responding: “That I don’t know. There is a number of questions that Lord Geidt has raised about the way that that system works and that is right that No 10 actually takes time to look at that and work that through. That will be for them to announce.”
He affirmed that Boris Johnson upholds the “highest standards required of his office”.
Shadow attorney general Emily Thornberry said the PM “doesn’t have a moral backbone”, following this week’s events.
She suggested to LBC that voters would be concerned about Geidt’s decision to quit: “I think people care that they have a Prime Minister who is dishonest and who is happy to break the rules and who doesn’t have a moral backbone, just doesn’t, and keeps losing ethics advisers because he is not ethical. I think that matters.”
She went on: “What is important is that we have a Prime Minister who doesn’t seem to think that the rules apply to him and they do.”
Liberal Democrat Leader Ed Davey said responding to the news: “Only a lying lawbreaker of a prime minister could think they could get away with not appointing an ethics adviser to hold his government account. Boris Johnson should not only appoint a new adviser but also give Parliament a say on who it is so he can’t appoint a crony to help let him off the hook.”