Seafarer union Nautilus International has been denied leave to appeal in a legal row over redundancies on OSVs controlled by bankrupt Greek group Toisa.
The UK Supreme Court said the appeal did not raise a point of law that "ought to be considered at this time."
In December, crew agency Seahorse Maritime won an appeal court case against Nautilus over the lay-offs.
Nautilus had brought legal action against Seahorse over redundancies in 2015, when some Toisa ships were moved to layup.
An employment tribunal had earlier ruled in the union's favour in 2016.
UK law states that employers must consult with an employee representative if 20 or more workers in "one establishment" are going to be dismissed.
Seahorse appealed on two points: whether the UK tribunal had jurisdiction and whether the ships constituted "one establishment" or whether each was a separate establishment.
The Supreme Court said: "The territorial jurisdiction point is potentially of general public importance but because of the establishment point this is not the right case in which to decide it."
Ships are separate establishments
The appeal court had ruled in December: "The importance of the establishment issue is that if each ship is a separate establishment it is very unlikely, though the facts have not yet been found, that at least 20 Seahorse employees would be liable to be made redundant on any one ship."
The tribunal previously found that in the previous 18 months Toisa vessels had been stationed in 11 countries, all but two of them outside Europe – in South-East Asia, the Caribbean, Latin America and Africa.
The appeal court found that each ship was "indeed an establishment."
"It is clearly a self-contained operating unit of the kind described in the case-law," it said.
Seahorse crew were assigned to particular ships, it said.
On the jurisdiction issue, it also ruled that the only connections with the UK are that some, though not all, of Seahorse's functions are performed through its agent, FMA, which is based in Farnham, UK.
"But in the case of the 1996 and 2010 Acts the fact that the employer is based in Great Britain is not by itself enough to overcome the territorial pull of the place of work," it added.
Nautilus' claim was dismissed for crew assigned to any Toisa ships stationed outside the UK.
