Conservative and Liberal Democrat MPs and peers will be given a free vote to change the law on assisted suicide.
Introduced by Lord Falconer of Thoroton, the former Labour lord chancellor, the private members bill allows two doctors to prescribe a lethal dose of drugs to a terminally-ill patient, who has less than six months to live.
Under the 1961 Suicide Act, helping someone to take their own life carries a maximum jail sentence of up to 14 years.
Supporters of assisted suicide, which is legal in Switzerland, say a formal legislative change is long overdue to clarify the law and reduce unnecessary suffering in the final weeks and months of people's lives.
But doctors, disability campaigners and churches have warned that a relaxation in the law could leave vulnerable people at risk, and damage the doctor-patient relationship.
A Ministry of Justice spokeswoman said: "The Government believes that any change to the law in this emotive and contentious area is an issue of individual conscience and a matter for Parliament to decide rather than government policy."
A spokesman for the campaign group Care Not Killing said: "Various attempts to change the law on assisted suicide and euthanasia have been debated and rejected by Parliament in recent years.
"Instead of repeating this process, we should be talking about how to ensure everyone has access to the care, drugs and treatment they need."
The bill already has the backing of senior MPs and peers, and ministers, including Liberal Democrat care minister, Norman Lamb. He said "the State should not stand in the way" of people determined to end their lives so long as there were strict safeguards in place.
He told Sky News: "Can we really be comfortable with a situation where people, acting out of compassion for a loved one who is dying, are left uncertain as to whether they will face prosecution?"
Four years ago the Director of Public Prosecutions issued guidelines for anyone 'acting out of compassion' for assisting someone's death would unlikely to be charged. No one has been prosecuted in the 90 cases examined since then. But supporters of the bill say it will help clarify the law.
David Cameron and Nick Clegg have rejected the idea in the past.
But it is believed more than a third of MPs would back a change in the law, a smaller group is strongly opposed, and up to 40% are undecided.
The proposed legislation will appear before parliament in the next few months.