Police have launched an investigation into a psychiatric hospital following staff complaints about the use of illegal drugs, it emerged last night.

Patients, predominately young men, are alleged to be leaving the Dorothy Patterson Psychiatric Hospital, in Walsall, to score drugs off dealers before returning to smoke them at the semi-secure unit.

Last night a spokesman for the local primary care trust, which funds the hospital, confirmed there was an investigation into drug use and possible dealing at the hospital.

One former nurse claimed the problem had become so widespread that patients no longer hid their drug habits.

Beverley Watson, aged 36, who quit as a psychiatric nurse last month, claimed managers had been aware of the situation since January. Ms Watson, who worked at the hospital for 18 months, added: "They frequently come back at all hours, even though they know the doors shut at 9pm.

"We do search patients and visitors' bags and pockets to a certain degree but it's very difficult to do that every time someone goes in or out."

In April, Walsall Primary Care Trust introduced a strict no smoking policy across its sites, doing away with 'smoking shelters'.

"It doesn't make sense - on the one hand our health is so valuable people are forced off site for a cigarette, but people are taking drugs on the wards," Ms Watson claimed.

The hospital, which opened in 1996, has two secure wards and two 'open' wards for voluntary patients who are free to leave unsupervised by staff.

Most patients are being treated for psychiatric conditions, including depression, schizophrenia, and drug-induced psychosis.

Mark Weaver, hospital director of mental health, said: "We have a duty of care to our patients. If they have a dual diagnosis we can treat their mental conditions and also help them with their drug usage."

Martin Turner, spokesman for Walsall PCT, admitted cannabis was "an intermittent problem" and that this was not unusual, especially when treating dual diagnosis patients with acute mental health problems and related addictions.

"There is an ongoing police investigation into drug use and possible dealing at Dorothy Patterson Psychiatric Hospital," he said.

"Cannabis is an intermittent problem in many mental health units, as a result of having dual diagnosis patients. This is an issue we take very seriously."

Mr Turner confirmed voluntary patients were free to leave unsupervised. He said" "Everybody in the NHS is subject to frustrations like this. The police investigation may go some way to address this one, but we're still looking for a long-term solution."