The NHS is failing to ensure all non-emergency hospital patients are kept in single-sex accommodation, the government has admitted.
Many people are uneasy about mixed sex wards
Campaigners say mixed-sex accommodation denies patients the chance of treatment with "privacy, respect and dignity".
Ministers were insisting as recently as November that 99% of patients were being seen in single-sex wards.
But the government has now announced that 28 NHS trusts - 15% of the total - need more help achieving this.
The promise to end mixed-sex accommodation was first made in the 1997 general election manifesto after Tony Blair attacked the Conservatives for failing to have "the wit" to end the practice.
A target was introduced in 2000 that within two years 95% of patients would be in single-sex accommodation.
The government's definition includes separate wards or bays divided by fixed partitions, which campaigners say is not enough. Patients also needed access to single-sex toilet facilities.
Departments such as intensive care and A&E do not have to fully comply for practical reasons, although they are expected to make an effort.
Last year, ministers said NHS managers had reported just 1% of patients were seen in mixed wards.
But the government launched an inquiry after reports from patients and a survey by the Healthcare Commission cast doubt on the claim.
Chief nursing officer Christine Beasley, who conducted the inquiry, has now concluded the NHS had to do more to keep male and female patients separate.
Her report called on all hospitals to review their practices and said 28 trusts were receiving support to change their practices because they were falling a long-way short.
It did not identify how many patients were staying in mixed accommodation, although campaigners said it ran into many thousands.
The report also failed to explain why NHS managers had given ministers what turned out to be incorrect information.
Professor Beasley said: "I am asking the NHS to do more to ensure that when there is no choice but to mix patients, that more safeguards are taken to ensure that privacy is maintained."
Health Secretary Patricia Hewitt said: "The NHS overall has an excellent record of treating people with dignity and respect.
"However, this report shows there is clearly still more work for the NHS to do to meet our commitment to eliminate mixed-sex accommodation wherever possible."
Katherine Murphy, of the Patients Association, said: "Patients have a right to be treated in a safe environment with privacy, respect and dignity.
"Patients continue to complain and for years politicians have promised to rectify this problem.
"After more than 10 years of promises broken by a succession of ministers, it is time for action."
The Patient Forums organisation said a survey it carried out of 2,500 patients in March this year found 25% had shared a ward or bay with members of the opposite sex.
Chair Jacquie Pearce-Gervis said: "We would like to see an eradication of mixed sex wards in order for all patients to spend their time in hospital recovering and not worrying that their dignity or privacy could be compromised."
Shadow health secretary Andrew Lansley accused the government of trying to bury bad news by publishing the report on the day Tony Blair was expected to announce his resignation.
"Labour will never learn that the public are fed up with their style of government instead of dealing with the problems being faced by patients."