Patients detained under the mental health act are often on mixed-sex wards where their safety may be seriously jeopardised, a key report warns.
Commissioners spoke to some 6,000 patients in private
The Mental Health Act Commission also found young people on adult wards, a practice ministers have vowed to end.
Detaining women and youngsters on mixed-sex adult wards is seen as likely to heighten their risk of sexual abuse.
Many of the 6,000 patients interviewed over two years found their hospital stay frightening, the report suggested.
The Mental Health Act Commission said while the standard of care it found during its survey was good, basic rights to safety and dignity were not always met.
It often found packed admission wards, where bed occupancy was over 100% and staffing levels inadequate.
"Patients admitted to hospital under compulsory powers should be able to expect that they will be cared for in a safe and welcoming environment," said Professor Chris Heginbotham, chief executive of the Mental Health Act Commission. "Sadly this is too often not the case."
"Staff working on psychiatric wards do a difficult job, and many of them do it very well. But too often we are shocked by what we see happening to patients in hospital."
Admission wards, particularly in London and other big cities, were "too busy", commissioners found.
Nearly 40% of the wards were running at over 100% occupancy when visits took place.
But the commission also expressed concern that "vulnerable" patients including women and young people were too often confined to mixed-sex, adult wards.
Data released in 2006 found a high incidence of rape, sexual assault and harassment in mental health units, while a report out late last year found children on such wards were often fearful of sexual harassment or aggression from adult patients.
There is currently no specific guidance on keeping mental health patients in mixed-sex wards.
The government promised to end mixed-sex accommodation in general by 2002, but that has still not been met, even though partitioning of wards into male and female areas is deemed to be acceptable.
Health Minister Lord Darzi said this week the NHS would have to be rebuilt if all wards were to become exclusively single sex.
Ministers have also promised to end the practice of keeping young people on adult mental health wards by the end of this year.
Health minister Ivan Lewis said progress had been made on giving mental health patients single sex accommodation.
But he admitted: "We know there is more to do."
Paul Farmer, chief executive of the charity Mind, was concerned by the report's findings.
He said: "Hospitals should be therapeutic places of safety but unfortunately for some people their hospital experience only serves to hinder rather than help their recovery.
"People with mental health problems are some of the most emotionally vulnerable patients in our hospitals and deserve the highest level of protection not the least."
Jane Harris, of the charity Rethink, said it was disturbing that the report found some wards were tougher and scarier than they were ten years ago, despite the significant investment in mental health services.
"The money is clearly not enough and doesn't seem to be getting through to the areas where it is needed most.
"It is unacceptable that in 2008 people in need of treatment are being forced to stay in dilapidated, overcrowded wards where they are at risk of sexual assault, and where they are lucky if they have a bed to sleep in."
The Mental Health Act was overhauled last year, driven in part by seveal high profile murders involving people suffering from mental health problems.
Marjorie Wallace of Sane said: "With reports like these, how can we proceed with implementing the new Mental Health Act if we are unable to ensure that those detained under its powers are treated and protected in humane conditions?"