He said the threat is constantly changing with al-Qaeda inspired Islamic extremists now plotting in smaller, harder to detect groups.

The danger is coming from an increasing number of hot spots around the world and there is also a growing threat from republican groups in Northern Ireland, who would attack mainland Britain if they could.

Mr Osborne, who is the senior national coordinator for counter terrorism, said: “On average we’ve probably had about one potential attack planned with an intent to create something similar to July 7 every year.

“The UK threat as we stand today remains at substantial, which means that a terrorist attack remains a strong possibility and could occur without warning.”