The company will take its Connemara ferry out of service from Monday (7 September), which is currently operating the two Portsmouth routes.
It follows previously advised closure of Brittany Ferries’ Portsmouth-St Malo route, which will also cease operating at the start of next week.
Poole-Cherbourg 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 said it wanted to apologise to all affected customers, and said it would do "all it can" to accommodate them on alternative sailings.
Brittany Ferries’ busiest route, Portsmouth-Caen, will operate as usual for the next two months, with ferries Normandie and Mont St Michel operating three daily departures as normal.
Its Armorique ferry, meanwhile, will cover the ships’ technical stops in November and December respectively, running in freight-only mode.
The schedule changes form part of the firm’s efforts to reduce costs as part of the company’s five-year Covid recovery plan.
It follows the UK government’s decision to impose quarantine restrictions on arrivals from Spain and France, which Brittany Ferries said led to "an immediate run on cancellations" with around 65,000 passengers having already cancelled reservations.
Demand has also "weakened significantly" for autumn, said Brittany Ferries.
“We carried virtually no passenger traffic between the months of April and June as the Covid-19 crisis hit,” said Brittany Ferries director general Christophe Mathieu.
“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.”
Brittany Ferries said reservations for the 2021 season were, however, strong, with around 100,000 more passengers reserving a holiday next year, compared with those booking during the same period in 2019.
