Around 90 firefighters were called to extinguish smouldering on board "La Perle" which is thought to have broken out in insulation "in a storage area used for food", a spokesman for the local maritime authority, Pierre-Louis Josselin, told AFP.

The incident was described as "combustion without flames" and all hotspots were extinguished by firefighters on Monday evening.

"There was no risk of radioactivity," Josselin added, explaining that the submarine had nuclear fuel on board but its reactor was not working.

The cause of the accident was unknown, with investigations being carried out by state-run submarine manufacturer Naval Group, which was in charge of the renovation work.

La Perle was first hit by a fire in 2020 while at port in Toulon, with the blaze raging for 14 hours and requiring 100 firefighters and 150 support crew members to bring it under control.

The submarine, which first entered service in 1993, is an attack vessel used for tracking ships, escorting aircraft carriers, carrying out coastal intelligence missions, and deploying special forces.

After the 2020 fire it was taken to the northern French port of Cherbourg for repairs, before returning to Toulon -- the headquarters of France's Mediterranean fleet -- in October 2020.

"This accident was not comparable to the one in 2020 which was an industrial fire, with major flames," Josselin explained.

He was unable to say if the latest incident would delay La Perle's return to active service, which was expected in the first half of 2023.

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