But few referrals to the anti-slavery agency triggered investigations into labour abuse at sites such as car washes and farms, officials said on Wednesday.

The hotline identified more than 7,100 potential victims and about 1,850 modern slavery cases last year - both up by almost half on 2017 - said Unseen, the charity that runs the service.

Labour abuse was the main form of exploitation, accounting for 54 percent of cases and three-quarters of victims recorded by the hotline, which receives calls from the public, police, activists, healthcare professionals and modern slaves directly.

Car washes, beauty and spa facilities, construction sites, hospitality and agriculture were the main hives of suspected modern slavery for callers to the helpline, according to Unseen.

Yet only about 10 of 320 possible labour abuse cases across these five sectors referred by Unseen to Britain's anti-slavery agency led to investigations - with one resulting in arrest - found data given exclusively to the Thomson Reuters Foundation.

"Even if we don't end up making arrests based on the referrals, the information is invaluable in helping us build up our intelligence picture," said Ian Waterfield, director of operations at the Gangmasters and Labour Abuse Authority (GLAA).

"The modern slavery helpline is a valuable tool in the fight against all forms of labour exploitation as it allows the public to report their concerns in a simple and timely way," he added.

The GLAA - which provided the data to the Thomson Reuters Foundation - received the biggest total number of slavery case referrals from Unseen last year - 468 - followed by the Metropolitan Police and the National Crime Agency (NCA).

The extent to which referrals by Unseen have prompted action across British law enforcement bodies is unclear, but the Metropolitan Police and NCA were not available to provide data on how many hotline tipoffs led to probes by their agencies.

As the number of slaves recorded by the government rises and convictions rates stall, activists and academics have questioned Britain's claim to be a world leader in the anti-slavery drive.

Britain is home to at least 136,000 modern slaves, according to the Global Slavery Index by rights group Walk Free Foundation - a figure 10 times higher than a government estimate from 2013.

Demand for data

Romania was by far the biggest source country, accounting for about a sixth for possible victims, followed by Vietnam, England, Poland and Bulgaria, said Unseen, which set up the hotline in 2016 backed by 1 million pounds in funding by Google.

The helpline for the first time last year also gathered data on exploiters - identifying more than 2,000 possible abusers from employers and recruiters to family members and partners.

"Focusing on exploiters will make a difference by feeding into law enforcement and prevention efforts ... and giving a better understanding of their methods and relationships with victims," said Justine Currell, executive director of Unseen.

Currell said the charity was slowly "getting better traction" with law enforcement referrals and resulting action.

The head of Unseen, Andrew Wallis, told the Thomson Reuters Foundation this month that efforts to boost data sharing and analysis between actors in Britain were "in their infancy."

With at least 7,000 suspected slavery victims uncovered in Britain last year - up a third on 2017 - according to data published last month, activists have raised concerns about the government's ability to support a growing number of survivors.

Britain is currently undergoing a review of its landmark 2015 law amid criticism that it is not being used fully to jail traffickers, drive firms to stop forced labour or help victims.