By Jane Dreaper
BBC News, Health correspondent
Lawyers are planning a High Court challenge to the proposed closure of a hospital which helps people with complex mental health problems.
The Henderson treats patients with complex needs
Solicitors acting for in-patients at the Henderson Hospital in Sutton, Surrey, said its closure would have a serious impact.
They estimate it would halve the number of beds available to people with personality disorder.
The NHS trust which runs the hospital was unaware of a legal challenge.
But the BBC understands that legal action to challenge the closure decision will begin in the next fortnight.
The Henderson Hospital does intensive work with people who have enduring emotional and behavioural problems, which can involve them harming themselves or others.
It used to receive income as a national service - but its referrals started to dwindle when funding was switched to local NHS trusts.
Mental health campaigners say the service is vital for deeply troubled people.
Lawyers claim there has been no consultation, and they want to press the case for a return to national funding.
Frances Swaine, from Leigh Day and Co, is a solicitor representing some of the 16 current in-patients.
She told the BBC: "We are aware at the moment that there are a total of 59 beds available in England and Wales for people with severe personality disorder problems.
"With the closure of the Henderson Hospital, which has 29 beds, that leaves us with just 30.
"It doesn't provide then for anyone with severe personality problems to have an in-patient bed.
"Removing 50% of those beds would throw those people back on to other areas of the NHS at much greater cost - because the Henderson has a good success rate."
South-west London and St George's mental health NHS trust, which ordered the closure of the hospital late last year, said formal consultations were still a possibility.
Peter Houghton, the chief executive, said: "The Trust is working closely with partners to develop new models - such as intensive short-stay beds - as part of a range of local community services for people with a personality disorder."
The Department of Health said new services for patients with personality disorder were being evaluated in many parts of England.
But Marjorie Wallace, of the mental health charity SANE, expressed concern that an "inhumane costing system" could force the closure of a unit which she called the country's only national resource for people with the most complex mental health needs.
"It is the equivalent of announcing the closure of the Royal Marsden Hospital for cancer patients or the Moorfield Eye Hospital.
"We can only hope that legal action to save the hospital proves successful."