The migrants were hiding in a refrigerated container, Dutch police said.
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At least 25 migrants have been found stowed away in a UK-bound ferry on Tuesday, Dutch police said.
The migrants were discovered aboard the Britannia Seaways cargo ferry after it had already left the Dutch port of Vlaardingen, prompting the vessel to return to its point of departure.
Police said the migrants had been hiding in a refrigerated container and that they are receiving medical care.
The origins of the migrants is not known, and images of the stowaways being led to busses show mainly young men.
Police told Dutch national broadcaster NOS that they had detained the driver and he was being questioned over his possible involvement in human trafficking.
The ferry had just embarked on a run across the North Sea to Felixstowe in southern England.
The discovery comes a month after 39 Vietnamese migrants were found dead in a refrigerated truck near London.