Devon and Cornwall Police said seven people were found in the back of the truck but could not confirm their age or nationality.
The Force Incident Manager, Insp. Martin Worthington, confirmed that the suspected migrants had been checked by ambulance teams and were in good shape.
He added that police were still determining how the truck entered the country and how people got to the vehicle.
Brittany Ferries offers services from Plymouth to and from Roscoff in France and Santander in Spain from Millbay.
Inspector Worthington said: “Our priority has been the welfare of these people. The ambulance service was there and helped with their medical evaluation. They were fed and watered.
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“We are liaising with Border Force to see what happens next.”
Live updates will appear in the blog below: