Peers will today begin a detailed review of the mental health bill, amid warnings the legislation faces defeat unless the government backs down on certain measures.
Among the proposals to be studied by the House of Lords are a series of cross-party amendments that critics argue are needed to stop patients being unnecessarily detained or forced to have treatment.
The bill would amend the 1983 Mental Health Act, and comes after eight years of consultations on how best to improve what is generally considered to be flawed legislation.
Ministers have argued that it provides more help for people with mental health problems, while at the same time ensuring doctors can admit patients for treatment or detain them where necessary, for example for public safety.
However, critics argue that a new power to detain people even where no real treatment is available, and where patients are not posing danger, is wrong. They want to ensure no-one is forced to have treatment unless it is of proven benefit to them.
Almost 100 MPs have so far signed an early day motion backing calls by the Mental Health Alliance, a grouping of 78 different charities and campaign groups, to reform the bill and ensure patients are properly protected.
A 2004 draft mental health bill had to be scrapped because of the overwhelming opposition, and last night Liberal Democrat peer Lord Carlile, who chaired the scrutiny committee into that bill, warned the new legislation faced a similar fate.
"It is time for the government to start listening. This bill will do nothing to bring mental health legislation into the 21st century. It will introduce a broad range of new powers, yet do little to make the public any safer," he said.
He added: "The government faces the very real prospect of defeats if it does not make major changes. A range of amendments with cross-party support have been tabled to ensure that the public get a bill which is fit for purpose."
However, health minister Rosie Winterton said the bill would reflect changes to mental health services in recent years, adding: "We have made it very clear in the bill that appropriate treatment has to be available for detention to take place."