Trade unions protested in Brussels on Monday as EU employment ministers met to discuss which rules should apply to workers posted abroad.
France is one member state which is pushing for tougher regulation.
Seconded workers often pay less social contributions and taxes than local employees.
Unions call this ‘social dumping’ and and say it is unfair competition.
“What we want is that the outsourcing is regulated, so we can check up on all these subcontractors,” said Constant Koumbounis, a Belgian union official.
Pierre Cuppens, the general secretary of Belgium’s construction workers union, said some workers “are experiencing slave-like working conditions fit for the Middle Ages.
“So we really demand to regulate and to hold accountable the entire supply chain and be able to punish those taking advantage of the situation,” he told euronews.
EU rules say workers may be posted abroad for up to two years for a specific job.
Their contracts must respect local labour laws, but social security payments are made to their home country.