Approximately 200 refugees stormed the northern French port of Calais on Saturday, with about 50 of them boarding the Spirit of Britain ferry.
A spokeswoman for ferry operator P&O Ferries said the migrants were unable to get inside the vessel from an external deck because the staff had locked the doors.
She said some of the refugees left the boat voluntarily and that the rest would be removed by police — by force, if necessary.
P&O Ferries said passengers could expect a delay of between 90 and 120 minutes because of what it called a "security incident."
Ferry company DFDS Seaways tweeted that the Calais port had been closed because of a "migrant invasion," adding that it would reopen “as soon as they are cleared.”
The incident followed a demonstration of about 2,000 people in Calais in support of migrants.
Thousands of refugees fleeing war and poverty in Africa and the Middle East have been living in tents in a vast slum near Calais dubbed “the Jungle" for the past 12 months.
It is used as a stopover point before they try to cross the English Channel to Britain in the hope of a better life.
FILE - A view of the migrant camp known as the new Jungle in Calais, northern France, Oct. 21, 2015.