Speaking in an unprecedented joint appearance before Parliament’s Intelligence and Security Committee, the heads of MI5, MI6 and the GCHQ answered questions in public.

The committee heard that Syria has become a haven for jihadists since the outbreak of the civil war, and hundreds of British citizens have travelled to the country as a result.

MI5 chief Andrew Parker said there are thousands of people living in Britain today who support al Qaida, and the head of MI6 Sir John Sawers warned the threat of terrorist attacks against Britain is increasing.

Asked how concerned he was about terrorist tourism, Mr Parker told the committee: “It has grown recently and is growing at the moment, because of Syria.

“Syria has become a very attractive place for people to go for that reason – those who support or sympathise with the al Qaida ideological message.

“We’ve seen low hundreds of people from this country go to Syria for periods and come back – some large numbers are still there – and get involved in fighting.

“This is partly because of the proximity of Syria and the ease of travel there, but also because it is attractive as what they would see as a jihadi cause.”

Mr Parker said: “It is a very important strand of the threat we face, the way in which there is interaction between people who live in this country who sympathise with or support the al Qaida ideology and then travel to areas where they meet these al Qaida groupings, either al Qaida itself in south Asia or some of these other groupings across other regions.

“The attractiveness to these groupings is that they meet British citizens who are willing to engage in terrorism and they task them to do so back at home, where they have higher impact, in this country.

“We’ve seen that played out in previous plots here, including 7/7.”

More here. H/T: Fjordman