Police sources said an address was also searched in the Ardennes region of eastern France in connection with the plot.

France remains on high alert over possible Islamist militant attacks.

More than 230 people have died in a series of assaults since the beginning of 2015 and the country has been under a state of emergency since November 2015.

An Egyptian man armed with two machetes allegedly tried to attack the Louvre museum in Paris last month. The man, named as Abdullah Hamamy, 29, was shot and critically injured after a confrontation with soldiers.

Bernard Cazeneuve, the prime minister, said: "We are facing an extremely high threat."

France's top constitutional court meanwhile overturned a law imposing prison sentences for consulting jihadist websites.

The law was introduced last June, seven months after the Paris attacks that killed 130 people. It set a two-year prison sentence for consulting jihadist websites regularly.

The 10-member court ruled that the law was unconstitutional because it infringed the freedom of communication unnecessarily and disproportionately.

A lawyer acting for a man serving a two-year sentence under the law had contested the ban.

The court said in a statement that law enforcement agencies had other resources to monitor websites inciting militancy and people who clearly had "a terrorist intention".