Female prisoners in England and Wales have been coerced into having sex with staff in return for favours such as alcohol and cigarettes, a report says.
The findings were published by the Commission on Sex in Prison, which was set up by the national charity The Howard League for Penal Reform.
Some inmates formed relationships as a source of comfort and support, it said, but some of these became abusive.
The Prison Service said it did not condone sex in prisons.
The Commission, which comprises leading academics, former prison governors and health experts, is the first independent review of sex behind bars in England and Wales.
'Protect the vulnerable'
The report said a form of sexual assault known as "decrotching" was believed to have occurred in jails - which involved female prisoners forcibly retrieving drugs from another inmate's body.
Women have different sexual health needs to men and they are at greater risk of entering prison with a sexually transmitted infection such as HIV, the paper also suggested.
Among its recommendations, the report said that "staff working with women in prison need training and guidance on how to support women, identify relationships between prisoners and recognise bullying".
It added: "The working relationships between prison staff and women can be beneficial and there is evidence that women prisoners are more open about their feelings than men and more likely to talk to staff about problems."
The commission said there was little reliable data available on consensual or coercive sexual activity in prisons.
It instead heard anecdotal evidence from agencies involved with the prison service, as well as from former and current prisoners.
The evidence included a study of women's health from 2006, when 220 female prisoners were interviewed a month after they arrived in custody. Eighteen said they had had sex with a man and 25 had had sex with a woman during that month.
The Prison and Probation Ombudsman also submitted that in the last five years it had investigated five female prisoner deaths, compared with one male death, in which sexual issues were found to have contributed in some way.
Chris Sheffield, chairman of the Commission on Sex in Prison, said: "Women in prison are particularly vulnerable and more likely than men to have a history of being a victim of violence or sexual abuse.
"It is important that policies recognise these differences and are developed in order to protect the vulnerable."
He added: "It is equally important that staff in women's prisons receive specific training on working with women."
A Prison Service spokeswoman said: "Sexual relations between prisoners are not commonplace. We do not condone sex in prisons or believe that prisoners in a relationship should share a cell.
"Reported incidents of sexual assault in prison are rare. Where an alleged sexual assault is reported or discovered it will be investigated and reported to the police if required."
The Howard League for Penal reform said it wanted to see less crime, safer communities and fewer people in prison.