Nine times as many people as was first thought will be forced to undergo compulsory mental health treatment under planned reforms, a study says.
New rules will introduce compulsory treatment
The draft Mental Health Bill proposes allowing patients to be made to take medication and detained if necessary.
The King's Fund study said that within 15 years as many as 13,000 could be placed under such orders, compared to the government's 1,450 estimate.
But the government said the "flawed" study produced incorrect estimates.
The bill was first introduced in 2002, in response to a perceived loophole in the law which allowed Michael Stone to kill Lin Russell and her daughter in a hammer attack in 1996.
Under the 1983 Mental Health Act patients can be sectioned, but only if their condition is treatable.
The new bill, which received an unprecedented 2,000 objections when it was first unveiled, proposes allowing people to be forcibly treated to protect the public.
Mental health campaigners and a joint House of Lords and House of Commons report have warned it will make it too easy to detain people, possibly even patients who have relatively mild personality disorders.
The King's Fund said there was likely to be a gradual, but sustained, increase in the use of community-based treatment orders for at least the next decade.
The study, based on evidence from other countries where similar laws have been introduced, also warned they were likely to be used differently from region to region.
A second, smaller-scale study, by the Cochrane Institute, a health research body, raised questions about how effective the orders would be, by comparing them with standard care.
It concluded: "Based on current evidence, community treatment orders may not be an effective alternative to standard care."
King's Fund report author Simon Lawton-Smith said: "At the heart of this lies a real challenge for mental health service commissioners and planners who will need to be prepared to meet the extra demand on their services."
And Richard Brook, of mental health charity Mind, warned: "We believe the bill could induce fear of compulsory treatment into the many ordinary people with mental health problems, driving them away from seeking the help they may desperately need."
Marjorie Wallace, head of Sane, added said the evidence from other countries showed it could be used a "more coercive, than compassionate measure".
Shadow Health Minister Tim Loughton said: "This system is unable to cope with this."
But a Department of Health spokeswoman said of the King's Fund study: "We believe that the lack of quality data and weakness in the analysis means that the estimates are flawed.
"For example, the report does not take into account changes to the bill that will limit use of community-based treatment to a defined group of patients. As the report does not consider this, the estimates are likely to be incorrect."