The government estimates that each case of slavery sets Britain back by 330,000 pounds

(Adds detail, quote from researcher)

By Kieran Guilbert

LONDON, July 31 (Thomson Reuters Foundation) - Britain is failing to grasp the scale, cost and impact of modern slavery, activists and researchers said on Tuesday, urging the government to invest in better data to tackle the growing crime.

The Home Office (interior ministry) on Monday revealed that modern slavery costs Britain up to 4.3 billion pounds ($5.7 billion) a year, but campaigners say the figure is likely to be higher with the government underestimating the number of slaves.

"This is likely to be seriously flawed as it is based on outdated estimates from 2014 of up to 13,000 people in slavery," said Jakub Sobik, a spokesman for Anti-Slavery International.

Britain may be home to as many as 136,000 slaves - trapped in forced labour and sexual exploitation - according to the 2018 Global Slavery Index by human rights group Walk Free Foundation.

This estimate is 10 times higher than the government's figure and points to emerging forms of slavery, such as the use of children as drug-runners, police and campaigners say.

"The figures in this (Walk Free) report are not based on UK statistics, but clearly this highlights the seriousness of this crime," a Home Office spokeswoman said in a statement.

The debate over data comes as Britain reviews its landmark 2015 anti-slavery law, amid criticism that it is not being utilised fully to jail traffickers, help victims or drive companies to tackle forced labour in their operations.

"Getting a handle on the true number of victims is vital," said Justine Currell, executive director of charity Unseen.

Walk Free's data - which counts 40 million slaves globally - has been criticised by some experts and academics as incomplete.

Ella Cockbain, a researcher at University College London, said she believed the 13,000 figure was still the best available estimate of the number of slaves in Britain, but called on the government to fund more detailed research as the crime evolves.

"Ultimately, there are massive gaps in the evidence base on modern slavery," Cockbain told the Thomson Reuters Foundation.

"One of the big issues is that modern slavery is a relatively hidden crime and those affected are hard to reach in traditional surveys."

The government estimates that each case of slavery sets Britain back by 330,000 pounds, from the cost of policing and providing support for victims to the impact of the physical and emotional harm on a survivor's quality of life and productivity.

Yet the 4.3 billion pound figure - which puts slavery second only to homicide in terms of its expense to Britain - does not include the cost to the criminal justice system, said Kate Roberts, head of office at the Human Trafficking Foundation.

Britain is unable to track progress in its fight against slavery as it does not know how much money it is spending, and has an 'incomplete picture' of the crime, according to a parliamentary committee and public spending watchdog. ($1 = 0.7613 pounds) (Reporting By Kieran Guilbert, Editing by Katy Migiro. (Please credit the Thomson Reuters Foundation, the charitable arm of Thomson Reuters, that covers humanitarian news, women's rights, trafficking, property rights, climate change and resilience. Visit http://news.trust.org)

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