OTTAWA -- Justin Trudeau is defending his government's restrictive approach to legalizing medical assistance in dying, arguing that it's a momentous change that needs to be implemented slowly and cautiously.

The prime minister made the argument Tuesday after some Liberal backbenchers expressed concern that the government's proposed new law does not comply with the charter of rights and falls short of what the Supreme Court ordered in last year's so-called Carter decision, when it struck down the ban on doctor-assisted death.

Trudeau acknowledged that critics have denounced the bill for failing to extend the right to an assisted death to mature minors or to individuals suffering solely from mental illnesses. Nor does it allow people diagnosed with competence-eroding conditions like dementia to make advance requests for medical help to end their lives.

These "very, very weighty questions" will take time to reflect upon properly, he said, but time is something the government, facing a June 6 deadline from the top court, doesn't have.

"This is the first step, it's a big one. Some people thought it should be bigger, I respect that," Trudeau said.

Still, he said he hopes MPs and senators will recognize "how momentous this step is and take it one step at a time as we move forward in a responsible manner because this is one of those things you can't undo."

But one Liberal backbencher, who got an earful over the weekend, told The Canadian Press the bill is much more restrictive than what her constituents want.

"Bottom line is I'd like to see some changes to suit my constituents' needs and to respect the Carter (decision) as a benchmark," Toronto MP Yasmin Ratansi said in an interview.

"At the moment, they feel ... that the bill has not met the benchmark. I don't want people who are suffering to go to court and to (have to) challenge to get their rights."

Ratansi, who represents Don Valley East, held a town hall meeting last weekend on the issue of assisted dying, conducted jointly with neighbouring Liberal MP Rob Oliphant, who co-chaired a special parliamentary committee that recommended a much more permissive approach than was adopted by the government in its new bill.

The Saturday meeting had been scheduled before the bill was introduced on Thursday but Ratansi was amazed to find that the roughly 200 people who attended, predominantly well-to-do seniors, had already dissected the bill and had passionate views on it. Some 80 to 85 per cent found it wanting, she said.

One, a non-verbal disabled woman in a wheelchair, used a computerized board to express "how annoyed she was with the bill" because it wouldn't allow her to make an advance request for an assisted death, Ratansi said.

"She said, 'I want advance directive because I am competent at the moment ... What happens if I am no longer able to communicate? What happens then?"'

Those who fear they may become incapacitated with dementia expressed similar concerns, Ratansi said. Others wanted the right to decide for themselves when a medical condition makes their lives intolerable.

"I guess people were saying, 'Who are you to decide what my pain level is and whether it's mental or physical?' I think we need to have thoughtful discussions on this."

In Carter, the Supreme Court said medical help in dying should be available to clearly consenting adults with "grievous and irremediable" medical conditions who are enduring physical or mental suffering that they find intolerable.

The government's more restrictive bill would require a person to be a consenting adult, at least 18 years of age, in "an advanced stage of irreversible decline" from a serious and incurable disease, illness or disability and for whom a natural death is "reasonably foreseeable."

Advocates of medically assisted dying believe the bill disregards the court ruling and have accused the Liberals, who profess to be the party of the charter, of authoring the kind of restrictive law they would have expected from the previous Conservative government.

"I would agree with them if the bill doesn't change and it's a whipped vote," said Toronto Liberal MP Adam Vaughan, parliamentary secretary to Trudeau.

However, he argued that the government is open to having a freewheeling debate on the bill and to making some changes, and it has vowed to let MPs vote freely.

"I think now you're looking at a process where Canadians, through their members of Parliament, can effect the changes they want to see ... That's the way government's supposed to work," Vaughan said.

Dominic LeBlanc, the government House leader, said the feasibility of amendments depends in part on how quickly the bill moves through the legislative process in both the Commons and Senate, given the push to enact the law before the June 6 deadline.

Some senators have vowed to propose amendments and LeBlanc said he hopes that will occur early so that "workable" changes can be incorporated into the bill before it's passed by the Commons. If the Senate were to vote for amendments after the bill is passed by the Commons, it would trigger a lengthy process of "ping pong," where the bill would bounce back and forth between the two houses of Parliament.