Hidden threat from al-Qaeda sleeper cells

Counter-terrorism police and Whitehall officials believe dozens of extremists could have arrived here by posing as students or legitimate visitors.

They are concerned both by the relatively lax checks that are made on the visitors before they arrive and by the ease with which they can outstay their visas without anyone noticing.

As many as 13,000 visa applicants may have entered the country from Pakistan in a seven month period since October last year without any checks on their supporting documentation.

The security services fear that because most do not mix with home grown terrorists, they are able to operate under the intelligence radar, acting as sleeper cells until ready to launch attacks in Britain.

Every year around 100,000 visitors arrive in Britain from Pakistan alone, which has been described by the Prime Minister as being part of a "crucible of terror" along with Afghanistan.