A lorry carrying 25 migrants in a refrigerated container has been found on a ferry travelling from the Netherlands to England.
Crew members were said to have discovered the migrants in a cool room on board the cargo ship.
It is believed there is a child among the migrants, although this is yet to be confirmed.
Emergency services flocked to the Britannia Seaways ferry which had departed from Vlaardingen this evening.
Up to 20 ambulances have been sent to the Vulcaanhaven port where the cargo ship has returned, NOS reports.
The migrants on board are all believed to be alive and receiving medical treatment.
Two people have been sent to hospital for extra care while the other 23 are being taken away by police.
The migrants were aboard a Britannia Seaways ferry owned by Danish operator DFDS that had just embarked on a run across the North Sea to Felixstowe in southern England.
Sniffer dogs were said to be used to comb the vessel for other possible stowaways.
“The crew discovered after a few kilometers that people were on board,” a police spokesman told De Telegraaf.
It turned around immediately and is now back at the harbour.
It remains unclear how the migrants came to be on board or what their nationalities are.
“Emergency services are reporting about 26 people,” a statement posted on the local government website said. “Word from the ship is that no one died.”
Their conditions remain unknown.
On October 23, 39 bodies, all believed to be Vietnamese migrants, were discovered in the back of a refrigerated truck near London.
Two people have been charged in Britain and eight in Vietnam over the deaths.
The container in which the bodies were found had arrived on a ferry from Zeebrugge in Belgium.