Parents should report their children to the police if they are at risk of becoming terrorists, Theresa May said today.

In the wake of the Tunisia atrocity, the Home Secretary warned it was difficult for the intelligence agencies and police to stop so-called ‘lone wolf’ attacks launched by individuals who have been radicalised online.

And she revealed plans for a major counter-terror exercise to ‘test’ the country’s ability to deal with an attack on British soil, after 40 plots were disrupted in the last decade.

Meanwhile, David Cameron asked for flags over Number 10 to be flown at half-mast ‘in memory of the victims of the terrorist attack in Tunisia’.