Changing the law on assisted dying could create "death squads" by putting the decision in the hands of doctors, a cancer specialist said.
Professor Karol Sikora told BBC's Newsnight that should the law come in, it would be at odds with providing good health care.
Peers will debate former Labour Lord Chancellor Lord Falconer's Bill on assisted dying in the House of Lords tomorrow when it comes before them for a second reading.
The bill proposes allowing doctors to prescribe a lethal dose to terminally ill patients judged to have less than six months to live.
Care Minister Norman Lamb also outlined his personal stance that people should be able to "make their own decision about their life", reiterating the stance he outlined in March that he would vote in favour of allowing assisted dying.
Mr Lamb - emphasising that he was speaking as an MP, not as a minister, as the issue would be decided by a free vote in Parliament - said he had come to his decision after " talking to an awful lot of people, people who've gone through the experience of a loved one dying", who had seen loved ones "going through months of pain and distress".
He told the programme: "Ultimately you have to ask the question, who should it be that decides? Should it be me, or anyone else in that situation, or should it be the state? Ultimately I think it's a very personal decision."
The Liberal Democrat said family members were put in an "invidious position" of not knowing whether they would be prosecuted for helping a loved one to end their life.
He added: "I think that there are very clear safeguards, the safeguards are absolutely critical, in a sense it was the fear of exploitation which always caused me concern in the past.
"But ultimately, should we stand in the way of someone wanting to make their own decision about their life, or should we set the safeguards in place to ensure that there is every chance of avoiding that exploitation?
"I'm very clear in my mind that the individual should be the person who decides, not the state."
Prof Sikora said warned against changing the law, saying: "I think the difficulty we have is that if you do implement the bill that's proposed on Friday, doctors are going to have to make the death decision, you're going to have to have essentially death squads, which is really out of the context of delivering good health care."
Prime Minister David Cameron yesterday spoke of his "worry" about legalising euthanasia, saying he was "not convinced that further steps need to be taken", and that "people might be being pushed into things that they don't actually want for themselves".
But former Archbishop of Canterbury Lord Carey earlier said he had changed his mind on the issue of assisted dying, after considering cases like that of locked-in syndrome sufferer Tony Nicklinson and "the reality of needless suffering".
A ComRes poll for ITV this week found that 70% of Britons would support allowing assisted dying under the measures being proposed, with just 10% disagreeing.
But some 47% said they believed legalising assisted suicide would "inevitably" lead to some vulnerable people opting to end their lives to avoid becoming a burden on their loved ones.
Writing in the Daily Mail, Baroness Sheila Hollins, former president of the British Medical Association, argued that legalising assisted dying risks letting "the genie out of the bottle" and normalising suicide.
She said current legislation had been called "a law with a stern face and a kind heart", adding: "We should remember the old saying, 'If it ain't broke, don't fix it'."