A desperate search was under way in Britain Thursday night for the terrorist bomb factory from which a suspected Al Qaeda cell planned to launch a devastating attack in Manchester, possibly as soon as Easter.

Hours after Britain’s most senior counter-terrorism officer resigned in disgrace, police operations were concentrating on a rundown block of apartments east of Liverpool's city center.

The block was cordoned off and large quantities of material were seized for examination. People were evacuated from the area, which had been rented out by a private landlord to foreign students.

Pakistan has criticized Britain’s immigration system and offered to help with security checks on foreign students after it emerged that the majority of those arrested in connection with the alleged terror plot entered the country on student visas.

British Prime Minister Gordon Brown Thursday pointed the finger at Pakistan, saying it needed to do more to stop suspected terrorists coming in to the U.K. But Wajid Shamsul Hasan, the country's High Commissioner to the U.K., hit back — saying Britain was not doing enough to prevent bogus applications for foreign student status. He added that Pakistani authorities could carry out background checks on those applying for student visas but were currently prevented from doing so.

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Eleven of the 12 arrested in raids across the northwest on Wednesday — including at Liverpool’s John Moores University — were Pakistani nationals, and 10 of them were in Britain on student visas.

The men are being questioned after raids that had to be rushed forward after Assistant Commissioner Bob Quick, the country’s most senior anti-terrorism officer, inadvertently disclosed details of the police plans.

Quick resigned as head of Special Operations at Scotland Yard Thursday morning, admitting that his bungle “could have compromised a major counter-terrorism operation."

Security sources are alarmed that terrorists may have exploited weaknesses in the vetting of student visa applicants to plant sleeper cells here.

Gordon Brown said that the police and intelligence services had thwarted “a very big plot”. He added that he would be raising his concerns with President Zardari of Pakistan.

The alleged Manchester plot represents a significant shift in tactics by terror groups based in Pakistan who view Britain as a prime target.

Click here to read more on this story from the Times of London.