After a passionate debate in the House of Commons, the lower house of parliament, 330 MPs voted in favour of the "Terminally Ill Adults (End of Life)" bill with 275 against.
Under the bill, mentally competent, terminally ill adults in England and Wales who are assessed by doctors to have six months or less left to live would be allowed the right to choose to end their lives with medical help.
Those in favour of the bill say it is about shortening the death of those who are terminally ill and giving them more control.
But opponents say vulnerable ill people may feel they should end their lives for fear of being a burden to their families and society, rather than for their own wellbeing.
Others expressed concern there had not been enough time to consider the bill before voting.
The initial parliamentary vote in favour the bill will start months of debate and the bill could be changed as it makes its way through both the House of Commons and the upper house of parliament, the House of Lords.
Kim Leadbeater, the Labour MP who introduced the bill, has said she expects the process to take a further six months.
The proposal has stirred a debate across the UK, with former prime ministers, faith leaders, medics, judges, the disabled and ministers in Prime Minister Keir Starmer's Labour government weighing in on the subject.
Polls suggest a majority of people in the UK back assisted dying.
Leadbeater said the law needed to catch up with public opinion.
