PIOTR was lured from Poland to Britain by a friend who promised to find him a good job. When he arrived, he found himself sharing a cold, dirty room with five others, and quickly realised this was a lie. Instead, he was put to work in a recycling plant by criminals who took away his passport and said he owed them money for transport, food and accommodation. A restaurant owner eventually offered Piotr a way out, only to make him wash dishes 60 hours a week for almost nothing. When Hope for Justice, a charity, found him, he was weak and malnourished.

The Home Office believes Britain is home to 10,000-13,000 “modern slaves”, by which it means people who are forced to work or who have been trafficked into the country for exploitation. Others put the figure much higher: the Walk Free Foundation, an anti-slavery group, reckons there could be as many as 136,000. The number of people identified by the National Crime Agency (NCA) as possible slaves has been rising fast, from 1,746 in 2013 to 5,145 last year, partly because of a new anti-slavery law passed in 2015.

Modern slavery used to be associated with criminal enterprises such as brothels and cannabis farms, whose illegality leaves workers fearful of reporting abuses. But the variety of businesses known to be using slave labour is broadening, with unpaid workers turning up in fields, factories and high streets across the country.

Vietnamese citizens are the nationality most commonly exploited for labour in Britain, according to the NCA. It says they comprised nearly a fifth of the people identified as potential victims of forced labour between 2015 and 2017. This is largely because of the widespread use of slave labour in Vietnamese-run nail salons, which have been found exploiting teenagers who have been smuggled from Vietnam to Britain in the backs of lorries.

Sometimes such workers are rescued from cannabis farms or other manicure bars, only to be “re-trafficked” by Vietnamese gangs based in Britain, which put them to work elsewhere. In January, for example, three people were jailed for forcing trafficked Vietnamese teenagers to work in nail bars around Britain. Two of their four victims had been re-trafficked after being taken into care.

Other nationalities end up in different industries. Romanians in forced labour often toil at hand car-washes. That business has become so common a centre of slavery that in June the NCA and other agencies launched Safe Car Wash, a smartphone app which helps motorists spot signs of exploitation. Workers in unsuitable clothing, living on-site and charging less than £6.70 ($8.77) per wash are among the giveaways. On July 30th Unseen, a charity, released a similar app which allows people to report suspected cases of slavery.

Different regions have their own problem industries. According to a report by the Gangmasters and Labour Abuse Authority, which regulates firms that supply agricultural workers, the most exploitative industry in Wales is shellfish picking, and in the past three years the nationality most commonly taken advantage of there is Sudanese. In Northern Ireland, the poultry and egg businesses are said to be the worst offenders.

A mixture of factors make certain industries magnets for slavery, says Kevin Hyland, the outgoing anti-slavery commissioner. Low-skilled work for which clients pay in cash makes it easier to keep workers off the books. And the lack of rules governing businesses like nail salons and car washes means less contact with officialdom. Even in industries that are more regulated, like agriculture and fast food, the authorities can be slow to enforce the rules, says Mr Hyland, allowing criminals to “sneak in”. Prosecutions are rare.

That is also because distinguishing between slavery and merely bad working conditions sometimes proves difficult. In February police raided a daffodil farm in Cornwall over concerns that 200-odd workers at the site were slaves. But no charges were brought. Indeed, on the night of the raid, some of the workers gathered outside the local police station to protest; they later put out a statement indignantly saying that they were not slaves at all, and demanding a public apology.

Identifying the real victims of slavery, such as Piotr, remains the challenge for the police. As some exploited workers continue to toil behind closed doors, others are labouring in plain sight, in industries whose customers might never suspect they were supporting slavery.