They backed an amendment delaying debate on the assisted-dying Bill for six months by 148 to 100, effectively preventing the private member's Bill sponsored by Lord Joffe becoming law in this session of Parliament.
But Lord Joffe told peers he would reintroduce his Bill in the next parliamentary year, raising the prospect of another full-scale debate on the issue.
The symbolic vote came after an impassioned eight-hour debate in which more than 90 peers spoke. Peers, led by the Archbishop of Canterbury, attacked Lord Joffe's Assisted Dying for the Terminally Ill Bill which would give doctors the power to prescribe a lethal dose of medication to allow terminally ill people to take their own lives.
The Bill, based on a law in the American state of Oregon, had little chance of becoming law but the debate and vote became a test of public opinion over the first moves towards legal euthanasia in Britain.
Opening the marathon debate, Lord Joffe insisted that his private member's Bill would give patients extra choice over the end of their lives. But critics said the terminally ill would come under pressure to die rather than use expensive pain-relieving care.
Lord Joffe said: "We can move forward on this sensitive matter with confidence, secure in the knowledge that the Bill would not impose anything on anybody and only provides an additional end-of-life option for terminally ill patients which they are free to accept or reject as they and only they decide."
But Lord Carlile of Berriew, the Liberal Democrat peer and proposer of the amendment designed to halt progress of the Bill, warned: "Despite protestations to the contrary, everyone in this House knows that those who are moving this Bill have the clear intention of it leading to voluntary euthanasia."
The Archbishop of Canterbury, the Most Rev Rowan Williams, launched an offensive against the Bill in an unprecedented joint letter with the Roman Catholic Archbishop of Westminster, Cardinal Cormac Murphy-O'Connor.
Dr Williams told peers: "Whether or not you believe that God enters into the consideration, it remains true that to specify even in the fairly broad terms of this Bill conditions under which it would be both reasonable and legal to end your life, is to say that certain kinds of life are not worth living.
"We would also jeopardise the security of the vulnerable by radically changing the relationship between patient and physician."
He was backed by the former archbishop of Canterbury Lord Carey of Clifton. Lord Carey warned: "If introduced, assisted suicide might be treated as casually as abortion is today, after a few years."
Lord St John of Fawsley, a Conservative peer and Roman Catholic, said: "The deadly sin of our time is not sexual promiscuity, about which the Church goes on too much. The evil of our time is greed. It goes throughout society and at every level. This Bill would open the way to abuse by the greedy and acquisitive, bringing pressure on those who are at their most vulnerable."
But Baroness Jay of Paddington, the former Labour leader of the Lords, backed the measure. She said: "However much we respect the opposition to this Bill in principle, we live today in a very diverse and predominantly secular society in which the importance of human rights is increasingly valued.
"We have to recognise some terminally ill people would prefer to end their lives in a controlled and dignified manner, rather than receive care until a so-called natural death."
Lord Gilmour of Craigmillar, a Tory, said: "Terminally ill patients are allowed to stop taking life-preserving drugs and thus probably face many days of pain before they die; and doctors are allowed to give pain-killing drugs, even though they know it will speed up the death of the patient."
He added: "This Bill is not a Rubicon and because I think the opponents of this Bill are being illogical and because public opinion is strongly in favour, I strongly support it."