A Dutch ferry crew found 25 people in a refrigerated container on a boat sailing to the UK and forced the vessel to return to port in the Netherlands, police and emergency services said.
Those found received medical attention at the port of Vlaardingen, near Rotterdam, with two being transferred to hospital for more care, they said.
"On board a ship it turned out that several people were found in a cooling container. The ship returned to the harbour," Rotterdam region emergency services said on Twitter.
"The 25 people were taken off the ship and given the medical care they need. Earlier the message came from the ship that no people died."
Police dogs were searching the ship, which was originally bound for the British port of Felixstowe, to check that no more migrants were on board, they added.
A large number of ambulances and other emergency vehicles could be seen near the busy port, Dutch media reported.
The boat was a cargo ferry run by the DFDS Seaways line, which said the migrants had come on board on a lorry trailer, De Telegraaf newspaper said.
The discovery comes a month after 39 Vietnamese people were found dead in a refrigerated truck in Essex.
Multiple people were arrested, including the truck driver from Northern Ireland, who was charged with 39 counts of manslaughter, conspiracy to traffic people, and money laundering.