Twenty-five migrants have been found in a refrigerated container on a ferry that was heading to Felixstowe from the Netherlands.
The Britannia Seaways ship returned to the port of Vlaardingen, near Rotterdam, after the stowaways were discovered.
Gert Jakobsen, vice president of communications with the ship's operator DFDS, said the migrants were found after they made a hole in the trailer.
The roll on, roll off ferry was not far off the Dutch coast en route to the Suffolk town when the discovery was made at around 5pm on Tuesday.
Mr Jakobsen said police were waiting at the port and the migrants were taken to hospital by ambulance to check for hypothermia.
"We don't know about the checks. Of course, they were not in a very good condition" he said.
It is not known the ages or sex of any of the migrants or where they are from.
Dutch police said 23 people were transferred to a police station after medical check-ups, while two were receiving extra care in hospital.
Dogs are searching for other migrants were still on the ship.
It comes weeks after the bodies of 39 Vietnamese migrants were found in a refrigerated lorry container in Essex after arriving on a ferry from Zeebrugge, Belgium.
And yesterday, eight people were found in the back of a lorry on the A14 in Cambridgeshire and a man was arrested on suspicion of assisting unlawful immigration.