ESA, a company established in Poland, carries on business in the electricity sector. It has a branch in Finland. In order to carry out electrical installation work at the construction site for a nuclear power station in Finland, ESA concluded, in Poland and under Polish law, employment contracts with 186 workers. The latter were posted to ESA’s Finnish branch. They were assigned to the construction site and were provided with accommodation. The workers argued that ESA did not pay them the minimum remuneration that was due to them under Finnish collective agreements for the electricity sector, which they argued were applicable under EU law. They individually assigned their pay claims to Sähköalojen, a Finnish trade union, so that it