Authorities in the Netherlands discovered 25 migrants hidden inside a truck aboard a cargo ferry en route to Britain shortly after it left the country on Tuesday.
The Britannia Seaways ferry quickly turned back to the port of Vlaardingen, just outside of Rotterdam, emergency services said.
The origins of the migrants are not yet known. The ferry is owned by Danish operator DFDS and had planned to embark across the North Sea to Felixstowe, in southern England.
The migrants had to return to the Dutch port. Most of them are reportedly male. Two of them were taken to hospital for treatment and the other 23 received medical assessments in the port before police took them in for questioning, according to a report posted by regional emergency services.