The true story of UN contractors' role in human trafficking and enforced prostitution is to be told in full for the first time in a new book by a former UN human rights investigator.

In The Whistleblower, published this week, Kathryn Bolkovac describes how she thought she was taking a well-paid job with DynCorp International, a private military contractor employed by the US State Department to recruit police officers to serve as UN peacekeepers in Bosnia. Instead, the former police officer from Nebraska sacrificed her career and fled from Bosnia in fear of her life.

DynCorp, based in Virginia, is one of the biggest contractors working for the US government. It has been paid billions of dollars to train police in Afghanistan and Iraq. The company vehemently denies Ms Bolkovac's allegations.

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Her story has been made into a film, also called The Whistleblower and starring Rachel Weisz. The actress describes Ms Bolkovac as "a remarkable woman who has had the courage to tell the truth and stand up for the victims of sex trafficking, putting her own life on the line".

Ms Bolkovac recalls how DynCorp staff were suspected of buying weapons and trafficking girls as young as 12 from the Serbian mafia. Demoted and then fired, she was warned by a colleague that her life was in danger. In June 2001, she successfully sued DynCorp for unfair dismissal.

"It would be easy to attribute the misbehaviour to isolated incidents by a couple of bad apples," she writes. "But the disappearance of files from human trafficking cases that implicated DynCorp personnel, the abrupt and unexplained cancellation of legitimate human rights investigations, men from around the globe getting away with buying and raping teenage girls – these are not isolated incidents and cannot be dismissed as the actions of rogue individuals."

Ms Bolkovac cites allegations of sexual assault and human rights violations, many involving DynCorp staff, by UN peacekeepers in more than a dozen countries. She claims the use of private contractors has resulted in "the creation of a band of mercenaries – a secretive, unregulated, well-paid, under-the-radar force that is larger than the US army. Worse, the framework for recruiting, enticing and managing this band is severely and dangerously flawed."

Last night, a spokeswoman insisted that DynCorp "takes the issue of human trafficking extremely seriously". She added: "Authors produce the most compelling storylines possible, and that process requires a mix of fiction and fact ... Ms Bolkovac's limited first-hand knowledge of our predecessor company ended a decade ago. She has no basis for making comments about DynCorp International as it exists today; any current generalisations that are drawn from the book are unfounded."

'The Whistleblower' by Kathryn Bolkovac with Cari Lynn is published by Palgrave Macmillan