Netherlands cops discovered 25 migrants hidden inside a truck aboard a cargo ferry en route to Britain shortly after it left the country.
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.
According to reports, the migrants had to return to the Dutch port.
There were no dead bodies discovered, the stowaways were given medical help and two were taken to hospital, although not in a critical state.
Most of the migrants 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, report said.
Dutch emergency services – including fire engines and police cars – were dispatched en masse to the port town of Vlaardingen, and the NOS reported that the police remained on board the vessel in search of other potential stowaways, using sniffer dogs.