Terror attacks cannot be completely prevented in a free society, the head of MI5 has said, as he warned plots were now harder to spot and being hatched in just days.

Andrew Parker said the country was now facing an intense threat from violent Islamist extremists who were devising plots at a tempo he had not seen before in his 34-year-career.

The director general of the Security Service made a rare public appearance to outline the severity of the threat after four jihadist attacks that have killed 36 people since March.

A dramatic increase in the menace this year had seen attackers hatch plots in a matter of only days, he said.

Mr Parker also warned that internet firms were inadvertently helping terrorists conspire and they now had an ethical responsibility to help stop these worst excesses of criminal behaviour.