In terms of the pact, the company “commits to guarantee… a sufficient volume of freight in case of a 'no deal' Brexit, whatever the circumstances, to ensure the transport of essential products such as medicines,” it said in a statement.
The contract concerns three existing ferry routes: between Caen and Portsmouth; Le Havre and Portsmouth; and Cherbourg and Poole.
“Our fleet and our maritime routes are sufficiently flexible to allow us to respond favourably to requests from the British government to guarantee the transport of essential products such as medicines,” said the company's chief Christophe Mathieu.
Brittany Ferries transports about 2.63 million people between France and Britain every year with 12 vessels.
With 20 days to go to the deadline for Britain to leave the European Union, the two parties have yet to come to a so-called divorce agreement.
France has started rehearsing customs checks that will be imposed if Britain leaves without a deal, having to inspect incoming goods for the first time since border controls were eradicated under a unifying Europe decades ago.
In a bid to ease potential trade disruptions, France has spent some 40 million euros and hired 700 extra customs officers.