PREVIEW: Hunting The Sex Traffickers, Channel 4

For the first time, a dedicated covert police unit has allowed cameras to film a secret investigation into a human trafficking gang.

Filmed in Brazil, the UK and Europe, the series provides a glimpse into a secret side of policing that we seldom see, and we meet the women who've been exploited to pay for the lavish lifestyle of the criminals who control them.

In this first episode: after a package of documents is posted anonymously through the door of a police station, in the middle of the night, a covert police unit is tipped off to an organised crime group who are sex-trafficking women from Brazil into the UK. With access to a range of secret tactics, from large-scale surveillance teams to undercover officers, this unit uses every tool in the box to work out how the gang operates.

The ultimate target is to bring down the head of the gang, a career criminal who's always managed to elude the police. At the heart of the operation are the women who are being exploited. In Brazil we hear the experience of a woman who had been trafficked to a UK brothel by a separate gang.

The full series will be available to watch or download for free on All 4, following transmission of this first episode on Monday 19 July.