Suspects being questioned today after one of the biggest anti-terror operations since the July 7 attacks exploited lax student visa regulations to enter the UK from Pakistan, Whitehall sources said yesterday.

As police continued searches in ­Liverpool, Manchester and Clitheroe, Lancashire, after the raids on Wednesday, the Home Office said student visa checks had been tightened in the last fortnight because of widespread abuses of the system.

There are concerns inside government and the security services that the 11 Pakistani nationals being held in the north of England could have gained entry on student visas in order to form a sleeper cell. Gordon Brown talked of the police having foiled a "very big terrorist plot".

The operation which led to the arrest of the men, along with one Briton who is said to have roots in the same tribal area, was rushed forward after the country's top anti-terror officer carried papers under his arm detailing the raids as he walked into 10 Downing Street in full view of photographers.