The route to Porto in northwest Portugal would probably be served from Plymouth in Devon, but Portsmouth is another possible departure point.
The journey could start before the end of May to capitalise on the surge of demand for Portugal – the only major European country on the government’s quarantine-free green list.
The link would be only for vehicles and their occupants – foot passengers would not be carried initially.
Brittany Ferries, based in Roscoff, France, already sails between Plymouth and Santander in northern Spain, about 150 miles from the Portuguese border.
But car drivers, motor-home owners and caravaners hoping to travel between the UK and Portugal currently face a tangle of red tape if they transit via Spain (and possibly France). They are also obliged to quarantine on their return home – even if they drive through Spain to the port at Santander without stopping.
A direct ferry link would bypass such problems. Portugal is expected to relax rules for British travellers arriving direct from the UK.
Porto, the second city of Portugal after the capital, Lisbon, is about one-third further from Plymouth by sea than Santander. It is possible that one sailing a week could operate to Porto and another to Santander.
The sailing time is around 30 hours, which could mean a late-evening departure and an early morning arrival two days later, or a morning departure and an afternoon arrival next day.
The vessel is expected to be Cap Finistère, launched in 2000, though the route may be allocated to Pont-Aven, which entered service in 2004.
Fares are likely to be around 25 per cent higher than on the existing Spanish link.
Travellers who do not have cars but who prefer not to fly will be disappointed by any exclusion of foot passengers.
As The Independent has reported, no destination on the UK’s green list is accessible except by flying.
The Brittany Ferries chief executive, Christophe Mathieu, said last week that many travellers prefer ferries to air travel: “There’s no mingling in a terminal and no need to travel in cramped conditions, in close proximity to others.”
A final decision on the ferry to Porto is expected to be taken early next week.
At present all holidays from the UK are illegal, but that ban is lifted on Monday 17 May.
Whether the route would continue when mainland Spain is placed on the green list – expected in June or July – will depend on bookings.
Besides being an attractive city in its own right, Porto is the gateway for the Douro Valley and the “natural parks” of northern Portugal. It is 300 miles from the Algarve coast in southern Portugal.
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