A law which would allow the enforced detention of people who are mentally ill even if they have not committed any crime is to be considered today by the House of Lords.

The new Bill also proposes strengthening powers made 23 years ago to ensure patients have therapy once they are released back into the community.

The changes, which would affect 14,000 of the 600,000 people who use mental health services each year, have come under fire from critics who say they fail to safeguard the rights of patients. About 50 Labour MPs have already signalled their intention to oppose the Bill.

Current laws do not allow people with severe personality disorders who have committed no offence to be detained.

Instead of replacing the old laws, the latest Bill - which would apply to England and Wales - proposes amending the existing Act. It was promised in the Queen’s Speech and comes after previous attempts to change the Act were thwarted by opposition from campaigners and doctors. The Government published a draft Mental Health Bill in 2002, but dropped it last March.

The desire to change the law has been driven by a number of cases in which people were killed by mentally ill patients, most notably that of Michael Stone, who in 1996 bludgeoned Lin and Megan Russell to death on a country lane in Kent. Stone was considered a psychopath and his condition untreatable, although a subsequent inquiry noted that clinicians were divided over his psychiatric condition.

But Professor Sheila Hollins, president of the Royal College of Psychiatrists, said that recent inquiries had not called for new laws but changes in services.

"Of course we are very concerned about public protection but this is supposed to be a health Bill and in a health Bill you would expect there to be some health benefit to people who are treated or detained compulsorily," she said, adding that the proposed measures risked stigmatising mental illness and driving patients underground.

"I’m very concerned about the victims but it’s balancing the victim and the patient," she told Radio 4’s Today programme. "I think the idea that somehow this Bill is going to prevent homicides and violent assaults is a mistake."

But Wendy Robinson, whose daughter was killed by a schizophrenic, said that more needed to be done to identify potentially dangerous people. She said: "In a lot of cases we are hearing through these inquiries, a lot of people are not picked up, they are not taken in and treated before something goes wrong.

"I do think there needs to be an added part of the law where they can actually take somebody in, section them and look after them.

"As the mother of a victim, the victim is not taken away and looked after - that person is taken to a mortuary. That is the reality for people who are having to deal with people who are killed unnecessarily."

Rosie Winterton, Health Minister, said that mental health services had improved significantly in recent years and the new Bill was to reflect modern provision. "We have made it very clear in the Bill that appropriate treatment has to be available for detention to take place," she said.

"Appropriate treatment has to be available. This is about getting treatment to people who need it."