Brittany Ferries will take its ship Connemara out of service from September 7, resulting in the closure of Portsmouth-Cherbourg and Portsmouth-Le Havre routes.
The operator said the schedule changes are “regrettable, but necessary” because of “a terrible summer season and weak forward demand for services this autumn”.
The cost-cutting measures follow UK government decisions to impose quarantine restrictions on those arriving from Spain and France.
About 65,000 passengers have cancelled reservations since quarantine measures were announced.
Christophe Mathieu, Brittany Ferries director general, said: “We carried virtually no passenger traffic between the months of April and June, as the Covid-19 crisis hit.
“When we resumed, we had hoped to salvage 350,000 passengers from a summer season that would usually achieve more than double that number.
“The reality however is that we are unlikely to reach 200,000. Passenger traffic accounts for around 75% of our income, so our bottom line has been hit hard.
“It’s is why we must continue to take decisive action to reduce our costs to get us through the worst of this unprecedented crisis.”
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The withdrawal of Connemara follows previously announced closure of the St-Malo Portsmouth route, with services terminating from 7 September.
Cherbourg-Poole will also remain closed for the remainder of the year, having ceased operations at the end of March and not having resumed in June.
The company’s busiest route Caen-Portsmouth remains unaffected for the next two months. Normandie and Mont St Michel will continue three daily departures as normal, with Armorique covering each ship’s technical stop in November and December respectively, running in freight-only mode.
Brittany Ferries said reservations for the 2021 season are strong. About 100,000 more passengers have reserved a holiday next year, compared with those booking during the same period in 2019.