Were parliament to back the bill, and see it through the full legislative process, Britain would follow other countries such as Australia, Canada and some U.S. states in launching what would be one of its biggest social reforms in a generation.
The "Terminally Ill Adults (End of Life)" bill would allow mentally competent, terminally ill adults in England and Wales, with six months or less left to live, the right to choose to end their lives with medical help.
The debate will start in the House of Commons, lower house of parliament, at 0930 GMT.
Those in favour of the bill argue it is about shortening the death of those who are terminally ill and giving them more control. Opponents believe vulnerable, ill people will feel they should end their lives rather than be a burden to their families.
What is set to be a passionate debate could help sway the many lawmakers who have said they are yet to make up their minds in the free vote, when politicians vote according to their conscience rather than along party lines.
Supporters and opponents of assisted dying are expected to protest outside parliament, reflecting the strength of feeling over a subject which has split the country 10 years after the last attempt to change the law was voted down.
Polls show that a majority of Britons back assisted dying and Labour lawmaker Kim Leadbeater, who has proposed the bill, says the law needs to catch up with public opinion. She says the bill includes "the strictest safeguards anywhere in the world".
But support in parliament appears less secure, with some lawmakers saying the current proposal lacks detail and needs to be underpinned by more research to study the legal and financial implications of a law change.
Critics warn that safeguards introduced around assisted dying in countries such as Canada have later been eased.
A group of lawmakers has lodged a proposed change, or amendment, which could stop the bill from progressing to a vote, although it will depend whether the speaker of the lower house decides to select it for consideration.
Prime Minister Keir Starmer has supported assisted dying in the past. He will vote on Friday but has not said how. His Labour Party is split over the matter.
If lawmakers vote in favour of the bill, it will proceed to the next stage of the parliamentary process, and face further votes in 2025.
Opponents could also attempt to "talk out" the bill so the debate ends without a vote.
