There are serious concerns over the standards of specialist care being provided to patients with the most complex mental health needs, a BBC investigation has found.
Patients sent by the NHS to stay in mental health rehabilitation units say they have been placed in unsafe environments, often far from home, with untrained staff.
Experts say not enough is being done to regulate the sector, which costs the NHS half a billion pounds a year.
The units, run by both NHS and independent providers, treat at least 3,500 patients each year considered too challenging for standard hospital settings.
They aim to offer a specialised approach, enabling patients to recover with skills to manage their conditions and re-enter the community. But some have remained there for 10 or more years, the BBC's File on 4 programme has found.