"Each one (will go) before a senior military council, " General Francois Lecointre told Le Parisian newspaper, and could be "delisted" or "put into immediate retirement".

The open letter, published by right-wing magazine Valeurs Actuelles last week, predicted that failure to act against "suburban hordes" -- or residents of mainly immigrant suburban areas -- and other groups who "scorn our country" will lead to "civil war" and deaths "in the thousands".

Prime Minister Jean Castex labelled the rare intervention in politics by military figures "an initiative against all of our republican principles, of honour and the duty of the army".

The 18, including four officers, were among hundreds of signatories to the open letter.

"I believe that the higher the responsibilities, the stronger the obligation of neutrality and exemplarity," said Lecointre.

The main instigators of the letter are alleged to have ties to far-right, anti-immigration movements in France.

The first signatory, Jean-Pierre Fabre-Bernadac, ran security in the 1990s for the National Front leader, the Canard Enchaine newspaper reported.

Others include retired general Antoine Martinez, who founded "Volontaires pour la France", a right-wing group committed to defending "traditional French values".

However, Lecointre denied that there was any far-right radicalisation in the army.

The letter went on to say, "laxist" government policies would result in chaos requiring "the intervention of our comrades on active duty in a perilous mission of protection of our civilisational values".