A British medic held for months in an internment camp for Tamil civilians has revealed how military guards dealt out cruel punishments, while many suspected of links to Tiger rebels were taken away and have not been seen since

Tamil women interned after escaping the horrors of the civil war in Sri Lanka were sexually abused by their guards who traded sex for food, a British medic has revealed.

Vany Kumar, who was locked up behind barbed wire in the Menik Farm refugee camp for four months, also claims prisoners were punished by being made to kneel for hours in the hot sun, and those suspected of links to the defeated Tamil Tigers were taken away and not seen again by their families.

Kumar, 25, from Essex, was released from internment in September, but has waited until now to reveal the full scale of her ordeal in the hope of avoiding reprisals against friends and family held with her. They have now been released after the Sri Lankan government bowed to international pressure this month and opened the camps.

The Sri Lankan government confirmed to the Observer that it had received reports from United Nations agencies of physical and sexual abuse within the camps, but maintained that it had not been possible to substantiate the allegations. It denied that prisoners had disappeared. In response, a UN spokesman accused Colombo of "doing everything it could" to obstruct attempts to monitor the welfare of the hundreds of thousands interned in the camps.

Kumar, a biomedical graduate, was incarcerated in May in what she describes as a "concentration camp", along with nearly 300,000 Tamil civilians who managed to escape the slaughter which accompanied the defeat of the Tamil Tiger rebels, who had been fighting for 25 years for a separate state on the island. Working amid heavy shelling in an improvised field hospital, she had spent months helping save the lives of hundreds of civilians wounded as they were caught between advancing government soldiers and the cornered Tigers.

Sri Lanka has consistently denied mistreating the detainees, but Kumar's damning new evidence will bolster the claims of human rights organisations which have repeatedly criticised the government in Colombo.

Speaking at the family home in Chingford, she accused the Sri Lankan government of persecuting Tamils as it sought to round up rebels who had escaped the fighting. "It was a concentration camp, where people were not even allowed to talk, not even allowed to go near the fences," she said. "They were kept from the outside world. The government didn't want people to tell what happened to them, about the missing or the disappearances or the sexual abuse. They didn't want anyone to know.

"Sexual abuse is something that was a common thing, that I personally saw. In the visitor area relatives would be the other side of the fence and we would be in the camp. Girls came to wait for their relatives and military officers would come and touch them, and that's something I saw.

"The girls usually didn't talk back to them, because they knew that in the camp if they talked anything could happen to them. It was quite open, everyone could see the military officers touching the girls," she said.

"Tamil girls usually don't talk about sexual abuse, they won't open their mouths about it, but I heard the officers were giving the women money or food in return for sex. These people were desperate for everything."

She said prisoners who complained about their treatment were singled out by the guards. "One time I saw an old man was waiting to visit the next camp and this military officer hit the old man. I don't know what the argument was, but the officer just hit him in the back.

"In the same area people were made to kneel down in the hot weather for arguing with the officers. Sometimes it lasted for hours."

Sometimes white vans appeared in the camp and took people away. White vans hold a particular terror in Sri Lanka, where they are associated with the abduction of thousands of people by death squads. "They were asking people to come in and take their names down if they had any sort of contact [with the Tamil Tigers]. They did an investigation and then a van would come in and they would take them away and nobody would know after that. I know people still searching for family members."

Kumar said that on arrival at the camp, near the northern town of Vavuniya, she was put in a large tent with several people she did not know. The camp was guarded by armed soldiers and ringed with high fences and rolls of razor wire. "The first two or three days I was alone there still scare me. When I arrived at the camp I put my bag down and just cried. That feeling still won't go. I just don't want to think about those two or three days in the camp, the fear about what was going to happen to me.

"For the first few days I didn't eat anything. We didn't know where to go to get food. I thought, 'Am I dreaming or is this really happening?' I never thought I would end up in a camp." Tens of thousands of people were crammed into flimsy tents which provided little respite from the intense heat. Toilets and washing facilities could not cope with the demands and food and water were in short supply.

"You have to bathe in an open area in front of others, which I find very uneasy. I stayed next to the police station, so every day I had a bath with the police officers looking at me, men and women. Everyone can see you when you are having a bath. So I would get up early in the morning about 3.30am, so it was dark," she said.

Kumar was held in the best-equipped part of the camp, but even there conditions were dire. "It is not a standard a human being can live in. The basic needs like water and food [were] always a problem. Most of the time you were queuing for water.

"The toilets were terrible, and there was not enough water, so we could not clean them. There were insects and flies everywhere. After two or three days of continuous rain, the sewage was floating on the water and going into the tents and everyone [was] walking through it, up to knee height." She was finally released into the custody of the British High Commission in early September.

The Sri Lankan government says it is aware of allegations of sexual abuse and punishment of prisoners, but denied large-scale abuse. Rajiva Wijesinha, the permanent secretary to the Ministry of Disaster Management and Human Rights, said "there was a lot of sex going on" inside the camp, but he claimed that most reports involved abuse by fellow detainees. "I can't tell you nothing happened, because I wasn't there," he said. "Individual aberrations could have happened, but our position is 'Please tell us and they will be looked into'."

He said he was aware of one report from a UN agency, but claimed that establishing the facts was very difficult. "We received a report that a soldier went into a tent at 11pm and came out at 3am. It could have been sex for pleasure, it could have been sex for favours, or it could have been a discussion on Ancient Greek philosophy, we don't know."

As many as 100,000 people are believed to have died in Sri Lanka's 25-year civil war.