France will create 1,500 new places in its jails to isolate Islamist extremists and prevent them from converting other prisoners, the prime minister announced as he unveiled a major new plan aimed at stopping young people becoming terrorists.

“Islamist radicalisation is a menace for our society,” said Edouard Philippe, unveiling the plan that includes measures on dealing with the hundreds of French men and women - and their children - who have returned from the battlefields of Syria and Iraq or are expected to do so.

School teachers will get more training on how to spot signs that their pupils are becoming radicalised, and local authorities will be told to improve their scrutiny of the small but growing number of private Islamic schools.

Mr Philippe travelled along with a dozen government ministers to the northern city of Lille to present the programme which comprises a total of 60 measures and puts the emphasis on prevention.

The plan is the third programme in less than four years to try to prevent and control the phenomenon, which many other European states are also grappling with, of disaffected young people turning to radical Islam and sometimes to terror.