The vulnerability of female patients on NHS psychiatric wards was revealed yesterday by figures showing that 435 patients reported being sexually assaulted last year, including 15 who said they had been raped.
The Tories, who obtained the figures, said the scale of assaults was increasing, raising new fears about the safety of patients. Labour had "broken their promises" on standards in mental health care, they said.
But the accuracy of the figures was questioned by Louis Appleby, the Government's mental health director. Previous reports of rape, when investigated, had turned out in the majority of cases not to have taken place, he said.
The figures, based on wards in England, are for reported assaults, not confirmed cases, and cover 52 of the 72 mental health trusts in England (72 per cent). Extrapolating them to England, the Tories say they show the number of reported rapes has increased 18 per cent, from 21 in 2005-06 to 25 in 2007-08.
Reports of sexual assaults have also risen from 590 to 653 across the country, up 11 per cent. Total assaults have fallen slightly, from 23,138 in 2005-06 to 22,322 in 2007-08.
The sexual safety of patients was made a key issue after an inquiry by the National Patient Safety Agency in 2006, which identified 122 incidents of sexual assault and 19 rapes over a two-year period.
But Professor Appleby said yesterday that investigations of the 19 alleged rapes had revealed that the majority were "very unlikely to have taken place".
"Only one out of the 19 was considered worth pursuing by the police," he said.
He added: "These are crimes and need to be taken very seriously. But the figures quoted in the past have proved completely unreliable. I can't say what the latest figures mean until they are properly examined."
Some patients who were seriously ill had delusions of being assaulted by people, or even machines and would wake up, find their clothes on the floor and conclude they had been raped, he said.
Andrew Lansley, Tory health spokesman, said: "It's shameful mental health hospitals have become places where patients live in fear for their safety. Labour have broken their promises to improve standards."