Volunteer firefighters were suspected of starting the devastating French forest fires.

Prosecutor Diane Ngomsik told the AFP news agency that the 18-year-old volunteer firefighter initially admitted that he had “lightly and lit the twigs on gasoline”.

BFMTV said a second 18-year-old man was also under formal investigation – a stage that could lead to formal charges and subsequent trials under French criminal procedure.

The Fontainebleau forest fire has partially closed the country’s main north-south highway since last Sunday.

President Macron said 10% of the forest had burned and praised the fact that there had been no casualties so far.

The forest is home to the Fontainebleau Palace and Park, a UNESCO World Heritage Site since 1981.

France has recorded nearly 11,000 fires since the beginning of the year.