A euthanasia bill was last debated, and defeated, in parliament in 2015 but public support for giving terminally ill people the choice to end their lives has since increased.
Before winning power in July, Prime Minister Keir Starmer promised to allow the House of Commons to revisit the issue, despite the concerns of church leaders and opponents who worry about any potential change.
Supporters say allowing assisted suicide would make some deaths more dignified and less painful, but critics argue it could lead some people to feel pressurised into ending their life.
Assisted suicide is currently banned in England, Wales and Northern Ireland, and carries a maximum prison sentence of 14 years.
In Scotland, which has a separate legal system and devolved powers to set its own health policy, it is not a specific criminal offence. But it can leave a person open to other charges, including murder.
The Terminally Ill Adults (End of Life) Bill would allow assisted suicide in England and Wales for adults with an incurable illness who have a life expectancy of less than six months and are able to take the substance that causes their death themselves.
Any patient's wish to die would have to be signed off by a judge and two doctors.
Choice
The measures are stricter than assisted dying laws in other European countries and also proposed legislation that is being considered in France.
A YouGov survey published last week indicated that 73 percent of Britons believed in principle that assisted dying should be legal. Thirteen percent were opposed.
The issue has received momentum in recent months by a campaign headed by high-profile UK TV personality Esther Rantzen, who has incurable lung cancer and has said she is considering going to Switzerland for assisted suicide.
The Swiss charity Dignitas claims to have supported 540 Britons over the past 20 years.
Supporters of the bill say UK citizens should not have to travel abroad to end their lives.
"It would give dying people a proper choice over how they die," said Sarah Wootton, of campaign group Dignity in Dying, which has held demonstrations outside parliament in support of changing the law.
The health and justice secretaries, whose departments would be at the forefront of implementing the law change, have publicly said they oppose the reform, although several ministers have spoken out in favour.
Former prime minister Gordon Brown is also against, as are 29 religious leaders of different faiths who penned a joint letter last weekend warning that a "right to die" could "all too easily" end in vulnerable people feeling they have a "duty to die".
Long opposed to the legislation because of fears over the impact it would have on doctors, the medical community has revised its stance. The main doctors' union the British Medical Association (BMA) now says it is "neutral".
Rushed?
Labour MP Kim Leadbeater, who has brought the bill, believes it is time for a renewed debate on assisted dying, but critics claim the reforms are being rushed through.
Lawmakers are expected to debate the bill for around five hours on Friday before likely taking a vote on whether to progress it to the next stage of the parliamentary process for further scrutiny.
MPS will get a free vote, allowing them to vote with their conscience rather than along party-political lines, and making the outcome difficult to predict.
Starmer voted for assisted dying at the last vote in 2015 but is not publicly taking a position this time round.
Approval would only mark the beginning of a legislative process that could take years. Similar attempts to change the law are being made in Scotland and the Isle of Man and Jersey -- self-governing British Crown Dependencies which are not part of the UK.
