Britains spy chief warns against terror threat from Muslim extremists

It remains the case that there are several thousand Islamist extremists here who see the British public as a target. Being on our radar does not necessarily mean being under our microscope, Parker said.

The reality of intelligence work in practice is that we only focus the most intense intrusive attention on a small number of cases at any one time. The challenge therefore concerns making choices between multiple and competing demands to give us the best chance of being in the right place at the right time to prevent terrorism, he added.

Addressing the Royal United Services Institute in Whitehall here,the espionage expert also stressed on the damage done by recent revelations by former US intelligence official turned whistleblower Edward Snowden.

GCHQ intelligence has played a vital role in stopping many of the terrorist plots that MI5 and the police have tackled in the past decade. It causes enormous damage to make public the reach and limits of GCHQ techniques. Such information hands the advantage to the terrorists, he said.