The diagnosis forces Mr Denton, 57, to withdraw from the campaign to legalise euthanasia to which he has been devoting his energy in recent years.

The Go Gentle Australia director – who set up that organisation specifically to achieve law reform around the country – may be absent at a critical juncture: as Premier Daniel Andrews' bill for assisted dying is introduced and thrashed out in the Victorian Parliament.

Go Gentle Australia's media director Gina McColl said Denton is "quite young and so the prognosis is extremely good".

"It's very successful surgery," she said. "He needs to have it quickly but after that he's expected to recover reasonably quickly and we're expecting him to join the campaign again in early September, some time like that.

"We're still in daily contact. He's still extremely funny and his humour is extremely black."

But Denton's absence from the campaign as the euthanasia debate heats up this month has been described by some proponents as a "disaster".

"It's terrible for Andrew, and a disaster for the broader campaign," one supporter of the bill said.

However, Go Gentle Australia played down concerns, saying Denton's work towards voluntary assisted dying would continue under its campaign manager Paul Price, a former senior adviser in the Baillieu Liberal government.

"The Go Gentle campaign continues in full force," Mr Price told Fairfax Media.

"In the next weeks and months we will be marshalling the support of the more than 75 per cent of Victorians who want voluntary assisted dying to become law."

Denton set up Go Gentle Australia last year – almost two decades after watching his father Kit die a slow and painful death from heart failure – in a bid to convince politicians to give terminally ill people the right to a physician-assisted death in strictly defined circumstances.

In that time, he has become one of the leading public faces of the "yes" campaign, appearing at community forums, across the airwaves, and alongside Victorian Health Minister Jill Hennessy to talk to delegates at Labor's state council.

But the debate is likely to intensify even further in coming weeks, when the bill on assisted dying is introduced in the lower house, paving the way for the most heated policy fight the Premier faces ahead of next year's election.

In a sign that the battlelines have well and truly been drawn, leaders of the Catholic, Anglican, Lutheran, Greek Orthodox and Coptic Orthodox churches placed an open letter in the Herald Sun on Monday saying that assisted suicide represented the "abandonment" of the terminally ill and sent a "confusing message" about the value of life.

Right to Life has also stepped up its opposition, sending out leaflets in nine marginal electorates – which looked as though they came from the sitting MP in each seat – suggesting Mr Andrews was attempting to sanction suicide to "save healthcare dollars".

- with AAP