London, Nov 28 : Britain’s counter-terrorism police believe that dozens of extremists could have arrived in the country here by posing as students or legitimate visitors, after13, 000 visa applicants 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 are also worried about arrivals from Somalia, Yemen and North Africa.

Police have discovered that the leader of an alleged plot to blow up shopping centres in Manchester last Easter ran a visa advice service in Peshawar, Pakistan.

He is thought to have helped other alleged members of his terrorist cell to arrive from Pakistan under the cover of student visas.

At least one arrived to attend a course at a "bogus college" that had already had its accreditation withdrawn.

Whitehall officials are also 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, The Telegraph reports.

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.

They are supposed to be checked by Home Office visa staff working in Abu Dhabi in the United Arab Emirates.

But according to an official watchdog, the Independent Monitor for Entry Clearance, many visa officers do not have enough time to go through applications carefully. (ANI)