Making his first speech as the Head of Britain's intelligence Agency, MI5, Andrew Parker says there are thousands of Islamic extremists living in the UK who see the British public as a valid target for attack.
According to a BBC report, the new head of MI5, also said Islamic terror had become more complicated to track than ever before.
"It remains the case that there are several thousand Islamist extremists here who see the British public as a legitimate target," Parker was quoted as telling a London gathering.
The BBC said, more urgent than the menace posed by those living in Britain, Parker had warned that terrorists from Pakistan and Yemen were "the most direct and immediate threats to the UK."
Andrew Parker, who has spent 30 years working for the agency previously directed MI5's counter-terrorism division at the time of the 7/7 bombings of London's transport system in 2005.
Over 300 convictions had been made on terrorism related charges in Britain since the September 11 attacks of 2001 and 31 March 2013, he said, but added that tracking terrorists and their online communication had been made far more challenging by the sheer number of online chat forums that could be used to pass messages and plan attacks.
Concerns over British jihadis in Syria
Parker added that MI5 was focusing particular emphasis on British citizens, who had traveled to Syria to join Islamist fighters there, saying that there was strong evidence that extremist Sunni groups in Syria had been aspiring to attack the UK along with other Western countries.
"This has long been a concern of Western governments - that British-based jihadists will one day return from the killing fields of Syria and turn their new-found skills on the population back home." The BBC reported Parker as saying.