Islamist fanatics have been taking guided tours of the Houses of Parliament in Westminster in order to examine the buildings, British intelligence has revealed, prompting calls for the Palace of Westminster to be closed to the public.
The information about the jihadists who joined tour groups in Parliament was disclosed to MPs at a security briefing, and then leaked to the Mail on Sunday by an intelligence official.
The news has also led to fears over security in the run up to next month's Remembrance Day service.
A heightened state of alert in the UK follows a shooting in the Canadian parliament in Ottawa, which led Labour MP Barry Sheerman to demand that Speaker John Bercow limit the number of tourists visiting the House of Commons.
Sheerman said: "When I raised this danger earlier this year I had my knuckles rapped. The former Commons Clerk Sir Robert Rogers accused me of being a scaremonger. But it does not just belong to the realms of Doctor Who for people to blow up Big Ben."
An aide to Bercow told the Mail he opposed the idea of closing Parliament to tourists, saying that it has tight enough security to prevent an attack like the one in Ottawa, during which one guard was killed by extremist Muslim covert Michael Zehaf-Bibeau.
"The day people would rather click on the internet to read about politics than come to Parliament to see it in action would be a dark day for democracy", the aide told reporters.
MPs have reportedly been instructed on what to do in an emergency situation involving a terrorist shooter within the walls of parliament.
Earlier this week former cabinet minister Peter Hain called for a review of the House of Commons' security procedures in the wake of the Canadian parliament shootings.
Among various other safety measures, some MPs are being advised to install panic buttons in their offices.
Police commissioner Sir Bernard Hogan-Howe warned last week that "up to five British jihadis" were joining militant group Isis (Islamic State) in Syria and Iraq every week. It is estimated that 500 Britons have travelled to the countries to join the fight.