A Portuguese submarine taking part in Nato exercises off Cornwall became caught in the nets of a French trawler, the Royal Navy said.
The submarine was training with the navy 34 miles (55km) off The Lizard on Tuesday.
It was below the surface when it became entangled in the nets of Daytona, a trawler registered to Saint-Brieuc, north west France.
There were no casualties and both vessels were safe, the navy said.
"The submarine immediately surfaced and made contact with the trawler," French maritime authorities said in a statement.
Falmouth Coastguards and the Maritime Accident Investigation Branch were made aware.
In 2004, five crew died when the French trawler Bugaled Breizh sank off The Lizard.
The families of the victims claim the trawler was dragged underwater by a submarine involved in an international military exercise, but courts ruled there is no supporting evidence.