British lawmakers on Friday approved plans to introduce medically assisted dying for terminally ill patients in England and Wales, advancing what would be one of the biggest social changes seen in Britain in decades.

After a debate that was at times emotive and fraught but remained respectful in tone, legislators supported the proposal by a vote of 314 to 291.

The vote on Friday was the second time lawmakers have approved the idea of medically assisted dying, after an initial vote in November was followed by months of scrutiny and debate in parliamentary committees. The issue has provoked deep division in and beyond the British Parliament.

The bill now goes to the unelected second chamber of the Parliament, the House of Lords. While the Lords can amend legislation, the fact that the bill has the support of elected lawmakers means that it is very likely to become law.

That would mean Britain would join a number of jurisdictions where assisted dying is permitted, including a small number of European countries, Canada and New Zealand, as well as 10 U.S. states and the District of Columbia.

It also makes Friday’s vote a consequential decision on changes that some have likened to Britain’s legalization of abortion in 1967 and the abolition of the death penalty in 1969.

The assisted dying legislation is deliberately narrow in scope. To qualify, patients would have to be over the age of 18, diagnosed with a terminal illness and have been given no more than six months to live. The lethal substance, provided by a doctor, would be administered by the person themselves.

The decision to go ahead would need approval by two doctors and a panel featuring a social worker, a senior legal figure such as a former judge, and a psychiatrist. A previous version of the plan would have put a judge in charge of the decision, but that idea was dropped because of the burden it would impose on the country’s already stretched court system.

The bill was introduced not by the British government, led by Prime Minister Keir Starmer, but by a backbench lawmaker from his Labour Party, Kim Leadbeater. Because it was regarded as an issue of conscience, lawmakers were permitted to vote as they saw fit rather than being instructed along party lines.

Months of scrutiny of the bill exposed sharp divisions, including in the cabinet, some of whose members publicly opposed it.

The bill passed by just 23 votes on Friday, significantly fewer than last year’s vote, when the majority was 55. The drop may reflect concerns expressed by some medical organizations, including the Royal College of Psychiatrists, the Royal College of Physicians and the Royal College of Pathologists, about the practicality of the legislation.

The Royal College of Psychiatrists said it could not support the bill and expressed concerns about a shortage of qualified psychiatrists to take part in assisted dying panels. “As things currently stand, mental health services simply do not have the resource required to meet a new range of demands,” the college said in a statement.

Supporters of assisted dying argue it curtails suffering in the final months of life. Under current law, Britons who help relatives or friends end their lives face potential prosecution. That means that terminally ill patients who decide to end their lives often do so alone.

“This is not a choice between living or dying; it is a choice for terminally ill people about how they die,” Ms. Leadbeater said. “Giving dying people choice about how they die is about compassion, control, dignity and bodily autonomy.”

Opponents regard the measure as a threat to vulnerable people with complex medical conditions, who might feel pressured to agree to a premature death, perhaps to end physical or financial burdens on their families. Others have argued that the money spent on administering a complex system of medically assisted dying would be better invested in broader and better palliative care. Another concern is that, once the principle is established, the criteria for an assisted death could expand.

Some critics oppose the legislation on religious grounds as a matter of principle, but in Friday’s debate most of those who argued against it cited practical problems with the bill, claiming it contained insufficient safeguards.

“There is no doubt that if this is passed in its current form, people will lose their lives who do not need to, and they will be amongst the most vulnerable and marginalized in our society,” said Diane Abbott, a Labour Party veteran and the longest serving elected female lawmaker.

Under parliamentary rules the bill will only return to the House of Commons if members of the House of Lords amend it. If that happens, further scrutiny in the Commons would be limited to the specific changes made by the second chamber.

In any event, assisted dying would not be available as an option for some time, with the changes expected to take as long as four years to implement.