The proposed closure of UK borders to low-skilled workers after Brexit risks driving vulnerable EU citizens into modern slavery, charities have said.

They warned the ban will lead to a boom in a black market for low-paid workers that will be exploited by criminals and lead to coercion and abuse.

The charities say it will harm EU citizens already in the country who may not know their legal rights post-Brexit and others lured by a glut of cash-in-hand jobs in restaurants, offices, farms and construction sites, who will risk being brought into the country by traffickers.

One charity, the East European Resource Centre, fears some businesses will be happy to hire undocumented workers if the supply of low-skilled workers is choked off by the immigration policy.

“Let’s not kid ourselves: someone will have to do the dirty, low-paid jobs, from construction to food processing to social care. Where are these workers going to come from?” said Barbara Drozdowicz, the centre’s chief executive.

“Even now, as eastern Europeans have full access to labour market, Poles and Romanians are two of five top nationalities who are reported victims of modern slavery in the UK. It is easy to imagine how increasing pressure to drive down labour costs [as the supply of low-skilled workers weakens] will translate into much more severe coercion and abuse.”

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Critics of the government’s proposed immigration laws say they do not make sense in an economy where there is near full employment.

Emily Kenway, a senior policy adviser at Focus on Labour Exploitation, said: “If you take away the legal routes, people are just going to fall into the hands of traffickers. If we think of the tragedy in October when 39 Vietnamese people were killed, you see the terrible consequences of that.

“This [immigration policy] is going to increase undocumented workers unless the government is going to introduce a way of people reporting exploitation without the risk of being deported. We know people won’t report exploitation and trafficking because of the fear that their data will be passed on to immigration..”

Nearly 7,000 people were referred to the National Crime Agency as potential victims of modern slavery in the UK in 2018. Boris Johnson’s predecessor as prime minister, Theresa May, vowed to tackle the issue.

Under the proposed new laws, only skilled workers will be allowed to settle in the UK. The government argues there are enough EU citizens in the country already to supply the low-skilled end of the workforce.

The home secretary, Priti Patel, has been mocked for suggesting employers could plug the gap with some of the 8 million Britons classed as economically inactive, a cohort that includes students, carers, long-term sick and retired people.

Among those at risk of exploitation will be workers who come into the UK on temporary farming visas. Fruit and vegetable farms rely heavily on EU citizens for back-breaking harvesting work, and the government has been criticised for pledging to allow in 10,000 people on seasonal agriculture work visas, when farmers say they need 70,000 a year to keep supermarket shelves stocked. Last autumn tonnes of crops were left to rot because of a lack of agricultural workers.

The Joint Council for the Welfare of Immigrants said the lack of supply of workers and the lure of over-staying for those who do secure a temporary visa would strengthen the hand of criminal gangmasters.

“People will come into the country legally and they might decide they like it here, fall in love or decide they can just earn more money for their families and will be lured into trafficked labour because they won’t have documents,” said Satbir Singh, the council’s chief executive.

The Home Office has been contacted for comment.