Brittany Ferries, which is the primary ferry operator at the port, will lay up two of its ferries, causing changes across its schedule.
It follows the government introducing quarantine measures on arrivals from France, which led to 35,000 ferry passengers either cancelling or delaying their travel plans.
Demand for bookings across autumn remain ‘extremely weak’, according to a statement from the company.
The firm’s director, Christophe Mathieu, said: ‘This is not something we want to do.
‘However, in the context of a terrible summer season we have no choice but to consolidate sailings that, by virtue of lack of passenger numbers, are uneconomic to run.
‘These extraordinary decisions are regrettable and we apologise in advance to all those whose travel plans will be disrupted.’
Brittany Ferries Bretagne, currently serving the Portsmouth to St Malo route, will be laid up from 7 September.
Brittany Ferries Etretat will not resume crossings as planned.
The ferry Connemara will continue to operate the Cherbourg and Le Havre rotations from Portsmouth, but will no longer serve Spain.
Brittany Ferries Armorique, serving the Plymouth to Roscoff route, will be laid up from 31 August.
Brittany Ferries Pont-Aven will continue the Plymouth to Roscoff route with three return trips per week from 10 September, and operate one return trip from Plymouth to Santander and from Roscoff to Cork in the week.
The company is working to accommodate displaced passengers on other services, and it requests passengers wait for notification of any change to their booking.
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