David Cameron says he is against assisted suicide because of the dangerous effects it could have on society.
The leader of the Conservatives said the law should not be changed to allow doctors and others to “accelerate death”.
Mr Cameron added: “I think the long-term consequences of permitting such action are too likely to be dangerous for society.”
His comments were made in a letter to pro-life campaigners three years ago during a previous attempt to weaken the law in Parliament.
A spokesman confirmed this week that his views have not changed.
Mr Cameron wrote: “I would not have voted for this or any Bill legalising euthanasia or assisted dying.”
He added that any change to legalise assisted suicide “may have a profound impact on the relationship of terminally ill patients with their doctors”.
In December Gordon Brown expressed his opposition to weakening the assisted suicide law.
He said: “I believe that it is necessary to ensure that there is never a case in this country where a sick or elderly person feels under pressure to agree to an assisted death or somehow feels it is the expected thing to do.
“That is why I have always opposed legislation for assisted deaths.”
Interim guidelines on what factors will be considered before someone is prosecuted for assisting suicide have been published today.
The Director of Public Prosecutions, Keir Starmer QC, was ordered by the Law Lords in July to produce a “custom-built policy statement” on assisted suicide.
But the law on assisted suicide can only be changed by Parliament. The House of Lords recently voted to keep the law unchanged.
There have been widespread concerns that any weakening of the law could put vulnerable people at risk.