David Cameron told the Commons he was concerned it would put pressure on frail and elderly people to end their lives if the bill proposed by Wolverhampton South West MP Rob Marris becomes law.
He told his weekly Prime Minister's Question Time: "I don't support the assisted dying proposals. I don't support euthanasia."
He said problems with the existing law can be 'dealt with sensitively' without 'bringing in euthanasia'.
Labour backbencher Mr Marris is adamant there will be safeguards, including the need for two doctors and a judge to agree before someone with less than six months to live can be given assistance to end their life.
But disability charity Scope warns it will put people under pressure.
Interim chief executive Mark Atkinson said: "Many disabled people are really worried about the legalisation of assisted suicide. "They are concerned that it will lead to them feeling under pressure to end their lives."
And Agnes Fletcher, director of Living and Dying Well, added: "The bill contains very few explicit safeguards."
Mr Marris said: "I think the safeguards in the bill will prevent anyone from being pressured. I have faith in judges and doctors.
"Guidelines on assisted dying were reviewed by the Director of Public Prosecutions. But I am a democrat. And it is for MPs to make the law."
An Express & Star online readers' poll found 69 per cent were in favour of legalising assisted dying and 31 per cent against.