A 62-year-old French man survived for 16 hours in an air bubble inside his capsized sailboat in the Atlantic Ocean.

He was rescued by Spanish coastguard divers in what they described as an operation "verging on the impossible".

The 40-foot (12 metre) Jeanne SOLO Sailor sent out a distress signal at 8.23pm on Monday, 14 miles from the Sisargas Islands off Spain's northwestern Galicia region, the coast guard said.

Tracking data show it had set sail from the Portuguese capital of Lisbon on the morning of the previous day.

As a rescue ship carrying five divers set sail, one of three helicopters sent to aid the search located the upturned vessel as the sun went down.

A diver was winched onto the ship's hull to seek signs of life and the man inside, who has not been named, responded to his banging on the hull by knocking from inside.

With the sea too rough to attempt a rescue, they attached buoyancy balloons to the ship's hull to prevent it from sinking further and waited until the morning.

Two divers swam under the boat to help the sailor out, who they found wearing a neoprene survival suit and submerged in water up to his knees.

Vicente Cobelo, a member of the coastguard's special operations team, told a local station the man voluntarily jumped into the freezing water and swam under the boat to reach the sea's surface.

"Of his own initiative, he got into the water and freedived out, helped by the divers who had to pull him through because it was difficult for him to get out in his suit," he said.

He was airlifted to safety and taken to hospital for checks but released soon afterwards with no issues.