As many as 800 North Korean "slave labourers" are working in the shipbuilding and construction sectors in Poland, in some cases for companies that are receiving financial support from the European Union.

A report by academics from the Leiden Asia Centre, working with lawyers and human rights activists, claims the North Korean workers' salaries are paid to managers and repatriated to Pyongyang.

The UN estimates that North Korea earns as much as £1.6 billion a year from labourers it sends overseas, with that money used to fund the regime's nuclear weapons and missile programmes.

The workers in Poland receive a "minimal livelihood allowance", claims the report, which was based on interviews with North Korean labourers and research into a network of joint-venture companies that is bringing the men into the EU.

The researchers have identified three North Korean firms - Korea Cholsan General Corp, Korea Rungrado General Trading Corp and Korea South-South Cooperation Corp - as providing labourers that are then assigned by two Polish companies, Alson Sp Zoo and Armex Sp Zoo, to pass on to firms that need cheap labour. Both Polish companies have North Korean nationals on their boards.

Among the employers are shipbuilders Nauta SA and Crist SA, both based in Gdnyia. In 2014, Polish labour standards officers investigated the death of a North Korean welder who was killed when his clothes caught fire at the Crist shipyard. The officials concluded that the victim, named as Chon Kyongsu, was wearing clothing that was flammable but had been provided by Armex.