Nigel Evans should not step down as Commons Deputy Speaker over allegations of rape and sexual assault, former chief whip Andrew Mitchell said.
Mr Mitchell, who quit the cabinet last year amid a storm about disputed claims that he called police officers "plebs" - said it would set a "terrible" precedent.
Tory MP Mr Evans was arrested at his home and questioned by police on Saturday but insists the claims by two young men are "completely false".
He made clear through his solicitor that he intended to carry on as Deputy Speaker while police continue their investigation. But Speaker John Bercow has agreed that he be excused his duties chairing the week-long debate on the Queen's Speech, which begins on Wednesday.
Defence Secretary Philip Hammond questioned whether Mr Evans could continue as Deputy Speaker while he was under investigation.
"I stick rigidly to the view that we should treat people as innocent until they are proven guilty but it is quite difficult to carry out a sensitive and high profile role while being under this kind of scrutiny," he told the BBC.
But Mr Mitchell told BBC Radio 4's Today: "If he were to resign now we would be in a terrible position where every public office holder would be open to blackmail and false stories in the media, triggering their resignation."
He went on: "Anyone who is caught up in the sort of intensive media storm which Nigel has faced, and which I and my family faced, obviously is in a very difficult position to carry out their job, But in this particular circumstance I am in no doubt that Nigel Evans should not resign and that he will be able to fulfil his duties as required by the Speaker.
"We have three deputy speakers in the House of Commons so if necessary there can be a degree of burden sharing.
"He has not been charged, he has not been found guilty, and we do still live in a country where you are supposed to be innocent until proven guilty."