The chief inspector of prisons in England and Wales says some jails are not providing inmates with condoms.
Sex is not allowed in prison but former prisoners and reform groups say it is widespread and mostly unprotected.
Prisons inspector Nick Hardwick says sexually transmitted infections (STIs) caught behind bars could be passed on to the general population.
Campaigners say all requests for contraception should be confidential and not disclosed even to prison staff.
One man, who asked not to be named, told Newsbeat he was locked up for more than three years and was sexually active but felt unable to ask for condoms.
"I was having a lot of gay sex in prison," he said. "I reckon more than 100 times and all of it was without a condom because I didn't want to go and ask for a condom.
"To ask for a condom would be outing myself and the person I was about to have sex with and I'd be breaking prison rules."
Even though sex between prisoners or inmates and staff is not allowed, prisoners are supposed to have free access to condoms if they are thought to be at risk of catching an STI.
"I would like to see which prisons they're available in because in the six prisons that I've been in, I've never heard that you can go to the health centre and get a condom," said the unnamed former prisoner.
"It's nonsense. How does someone know they're made available if they don't talk about it?"
The government says it believes that prisoners in a relationship should not share a cell.
It insists that sex is not common in jails but that condoms are available to all prisoners from qualified members of the healthcare team.
The World Health Organization (WHO) says HIV and Aids are serious problems for jail populations across Europe.
A 2006 joint report by the Penal Reform Trust and the National AIDS Trust said there was evidence that significant levels of HIV had been going undiagnosed in prisons.
Nobody from the government would give Newsbeat an interview but former Conservative prisons minister, Crispin Blunt, said the government had more important things to worry about.
He said the data that he had seen did not suggest there was a serious health problem in prisons due to unsafe sex.
The Howard League for Penal Reform announced it was spending two years looking at all elements of sex in prisons in England and Wales, from consensual to coercive sex in jails, in June 2012.