Some 25 stowaways have been found in a refrigerated container on a ferry bound for the UK from the Netherlands.
They were found at around 19:00 (18:00 GMT) by the crew; the ship then returned to the port of Vlaardingen near Rotterdam.
The Danish-registered ferry had been en route to Felixstowe.
The incident comes just weeks after the bodies of 39 people were found in a refrigerated lorry container in Essex in eastern England.
The stowaways received medical attention at the port of Vlaardingen.
Two of them have been taken to hospital to receive extra medical care, while the other 23 people have been transferred to a police facility after getting a medical check-up, reports the BBC’s Anna Holligan.
The bodies found in the Essex container last month were that of Vietnamese nationals and had arrived on a ferry from Zeebrugge in Belgium.
Two lorry drivers have since been charged with manslaughter, and several other men have been arrested in connection with the case.