In August 2014, then Home Secretary, Theresa May announced the terror threat was being raised to severe in the wake of warnings of the deadly threat posed by British jihadists radicalised fighting for Islamic State extremists in Syria and Iraq.

It remained at this level until the Manchester attack last month. Despite terror attacks being highly likely there had been no significant attacks on UK soil for more than a decade until the attack at Westminster in March of this year.

Speaking on the BBC's Today programme in the wake of the attack at Parliament, Dominic Grieve, Chair of the Intelligence and Security Committee, said that the security services have foiled 12 attacks in the past 18 months.

He said: "We've been extraordinarily fortunate. It's really been a miracle that it [a terrorist attack] hasn't happened sooner."

However, after the attack in Manchester that has so far left 22 dead and many more injured, Theresa May has now raised the terror threat in the UK to its highest possible level.