One in four men in South Africa have admitted to rape and many confess to attacking more than one victim, according to a study that exposes the country's endemic culture of sexual violence.

Three out of four rapists first attacked while still in their teens, the study found. One in 20 men said they had raped a woman or girl in the last year.

South Africa is notorious for having one of the highest levels of rape in the world. Only a fraction are reported, and only a fraction of those lead to a conviction.

The study into rape and HIV, by the country's Medical Research Council (MRC), asked men to tap their answers into a Palm Pilot device to guarantee anonymity. The method appears to have produced some unusually frank responses.

Professor Rachel Jewkes of the MRC, who carried out the research, said: "We have a very, very high prevalence of rape in South Africa. I think it is down to ideas about masculinity based on gender hierarchy and the sexual entitlement of men. It's rooted in an African ideal of manhood."

Jewkes and her colleagues interviewed a representative sample of 1,738 men in South Africa's Eastern Cape and KwaZulu-Natal provinces.

Of those surveyed, 28% said they had raped a woman or girl, and 3% said they had raped a man or boy. Almost half who said they had carried out a rape admitted they had done so more than once, with 73% saying they had carried out their first assault before the age of 20.

The study, which had British funding, also found that men who are physically violent towards women are twice as likely to be HIV-positive. They are also more likely to pay for sex and to not use condoms.

Any woman raped by a man over the age of 25 has a one in four chance of her attacker being HIV-positive.

One in 10 men said they had been forced to have sex with another man. Many find it difficult to report such attacks to the police in subcultures where the concept of homosexuality is taboo.

South Africa's government has been repeatedly criticised for failing to address the crisis. Only 7% of reported rapes are estimated to lead to a conviction. Jewkes said: "There's been a lot of concern about the way the criminal justice system works, because it's still woeful."

Before his election as president, Jacob Zuma stood trial for the rape of a family friend. His supporters demonstrated at the court house, verbally attacked his accuser and sang "burn the bitch, burn the bitch". Zuma was eventually acquitted.

Jewkes added: "The social space for debating these gender issues is now smaller than it was a few years ago. We need our government to show political leadership in changing attitudes. We need South African men, from the top to the grassroots, to take responsibility."

Anti-rape campaigners said the shocking figures demonstrated the need for reform. Dean Peacock, co-director of the Sonke Gender Justice project, said: "We need to make sure the criminal justice system is held to account. We have lots of discussion in this country, but not enough action is taken to ensure that perpetrators will face consequences."

Zuma, a polygamist, was criticised for emphasising his Zulu tribal identity and singing militant songs during this year's election campaign. He made comments that outraged anti-Aids and gender campaigners.

Peacock added: "We're at a complicated moment in South African history with revived traditionalism and there's a danger of gender transformation being lost.

"We hear men saying, 'If Jacob Zuma can have many wives, I can have many girlfriends.' The hyper-masculine rhetoric of the Zuma campaign is going to set back our work in challenging the old model of masculinity."

Carrie Shelver, an activist with People Opposing Women Abuse, said: "Generally there's a deficit of understanding and commitment to women's rights by the leadership of this country. It's simply not on people's agenda."

A report published by the trade union Solidarity earlier this month said that one child is raped in South Africa every three minutes, with 88% of rapes going unreported. It found that levels of child abuse in South Africa are increasing rapidly.