In March 1998 I was having dinner with staff from the UN in Sarajevo. The topic of conversation turned to the increasing number of bars in Bosnia, housing prostitutes from countries in eastern Europe, mainly Romania and Moldova. Someone said that in the infamous Arizona Market, near Brcko in north-eastern Bosnia, young girls from these countries were paraded weekly for sale and purchased by bar owners who put them to work as sex slaves.

It wasn't until the following year that I made it back to Bosnia. It became apparent that while the UN mission in Bosnia said it was "deeply concerned" about the rapid increase in trafficked women for use as sex slaves in Bosnia, some of its employees were heavily involved in providing a market for this trade. Every young girl I spoke to in these bars told me that, of the six to eight men they were forced to sleep with each night, the majority were UN employees.

At the same time I became friends with Kathy Bolkovac, an American police officer working for the UN who was outspoken about its complicity in the problem. She was accused by the UN in Sarajevo of "falsifying her timesheets". She later filed a case for unfair dismissal and won. It was evident that she had been considered too dangerous to keep on, as she was willing to reveal that very senior levels of the UN mission were involved in this sickening trade.

I was approached by a Bosnian photographer at around the same time who told me that he had photographed a very senior member of the UN one night in a bar with a very young Romanian girl. He was followed home, beaten up and had his camera stolen. Returning to my apartment that night I received an anonymous phone call saying "don't continue with your investigations or you will find yourself in trouble".

In 2002 I was sent to Liberia on a photographic assignment. The camps I was working in had been at the centre of a joint UNHCR-Save The Children investigation into sexual exploitation by UN troops. The findings had been so horrific that the UN was forced to submit to an independent investigation by the UN watchdog. This investigation played down the initial findings and claimed that the problem was not as widespread as originally thought.

February 2004 found me on a photographic assignment in the camp for internally displaced people in Bunia, Congo, next to the UN base. After only four hours in the camp I noticed there were holes in the perimeter fence separating the camp from the UN military base. An investigation revealed young girls from the camp, many of them victims of sexual violence by local militias, would cross over the fence each night to sleep with soldiers stationed there - often for a banana or a bag of peanuts.

When it became evident the UN was reluctant to act on the information I had given it, I decided I had no choice but to publish. Only then did the UN say it was going to start a full investigation.

Returning to DRC in July, I planned a follow-up on the story in Bunia. I became aware that the problem was not just there but was endemic to every town where the UN was based in DRC - and that the UN had first received reports of abuse as far back as 2002. These reports, filed to Kinshasa, had been buried and no action had been taken.

Before departing for Bunia, I received two phone calls warning me "not to return to Bunia", as well as a note delivered to my hotel saying: "If you continue your investigations against the UN there will be trouble for you." This note later disappeared from my hotel room.

I was also approached by several people in the UN who were increasingly horrified as to how widespread the problem was and how so much of the information was apparently being covered up. With long UN careers behind them they were risking their jobs to give me information, but felt that the levels of abuse and corruption had to be exposed if the UN was to continue to function with any degree of integrity.