The chief executive in charge of Tolworth Hospital has apologised after a Channel 4 Dispatches programme revealed appalling conditions at its mental health unit.

The South West London and St George's Mental Health Trust, and the other two trusts filmed, unsuccessfully tried to convince Channel 4 not to show the programme, citing fears it would identify vulnerable patients. But the film, entitled Dispatches: Britain's Mental Health Scandal, was shown on Monday night.

The trust's chief executive Peter Houghton said: "I would like to apologise on behalf of the trust to service users and their families for lapses in quality of care seen on the film. These will be investigated and we will take appropriate action.

"My top priority since becoming chief executive in July is to ensure our services are safe, effective, of a high quality and continually improving. While our staffing levels are in line with those across London we are committed to improving them."

Issues raised by reporter Janey Ayoade after her six months spent at Tolworth and the two other hospitals included women being forced to mix with men in communal areas, sexual harassment and assault, overstretched staff and illegally-administered medication.

Chief executive Mind, Paul Farmer, said: "It's hard to believe that the wards people go to for support when they are at their most unwell are often scary and dangerous places. Many mental health wards don't provide a safe environment, let alone a therapeutic one."

Another woman, who did not wish to be named, said inpatient services at the hospital in Red Lion Road, Tolworth, were "disgusting" and she had seen examples of filthy wards.

Mr Houghton added: "Filming for the programme was undertaken over a few weeks in 2005. We have begun a major training programme for staff to improve their ability to assess the risks of patients.

"The privacy, dignity and safety of all service users is of great importance to me. All wards have separate male and female sleeping areas, toilets and bathroom facilities. We support patients in reporting any assaults to the police."

It is not the first time the unit has been criticised. In March 2000 it was announced control of mental health services was to be moved from the now defunct Kingston and District Community Health Trust to St George's. An internal inquiry held trust managers largely responsible for a seriously flawed culture of care for Alzheimer's patients on Fuschias Ward.

An independent inquiry by the London NHS Executive followed soon after, which called for an overhaul of complaints procedures, spot checks on wards at night and on weekends, training for senior managers and better reporting of untoward incidents.