Unions have launched a legal challenge to defend their right to strike against employers attempting to relocate their operations elsewhere in the EU.
The case pits workers' rights under EU law against the principle of freedom of establishment for employers, reports the Financial Times.
In June, a High Court judge granted an injunction to Viking Line, a Finnish shipping company, to prevent strike action or boycotts by its unionised workforce.
The union was opposed to the company's plan to reflag one of its ferries as an Estonian vessel, so that it could be manned by a cheaper crew.
If the High Court decision is upheld, the International Transport Workers' Federation said there would be "very serious consequences" for workers' rights across Europe.
The unions said the implications of the High Court decision extend well beyond the shipping industry.
John Monks, general secretary of the European Trade Union Confederation, argued that the ruling would mean that "with a little careful planning, employers wanting to replace their workers with others from another member state who are prepared to be paid less can do so without the risk of industrial action".