The 12 men arrested during the raids in Manchester, Merseyside and Lancashire on Wednesday remain in police custody and are still being questioned, said a spokesman for Greater Manchester police.
Meanwhile, the revelations regarding the nationality of the suspects have stoked concerns that the authorities have lost the ability to control Britain's border security.
In particular, critics have pointed to alleged deficiencies within the student visa system, and the fact that of the 12 suspects, 10 were in the UK on student visas.
Home Office figures for between April 2004 and April 2008 - the last year for which figures are available - show that about 42,000 Pakistani nationals entered the UK on student visas.
Conservative shadow home secretary Chris Grayling called on the government to "urgently step up" background checks on students coming to Britain from countries which have been linked to terror.
The chairman of immigration campaign group Migrationwatch UK, Sir Andrew Green, said student visas were a "gaping hole" in Britain's borders.
Pakistan's High Commissioner to the UK: 'You can't blame Pakistan''
"Applicants from countries of concern like Pakistan and North Africa should be given a full interview by a UK-based visa officer and only admitted if they can demonstrate that they are genuine," he said.
"Last year over 10,000 students were admitted from Pakistan with what are clearly inadequate checks."
Former London Mayor, Ken Livingstone, told BBC Radio 4's Any Questions programme that even if the visa system was changed, foreign nationals would find a way to get into Britain.
"This city [London] makes billions of pounds a year out of selling further and higher education to people from abroad. They will find a way round it. If it isn't students coming in, it will be tourists.
"We catch these people because of electronic surveillance and double agents and we aren't going to pick one up easily at the border," he said.
However the government insisted that the student visa system had already been been tightened up.
The Home Office said Pakistani nationals who applied for UK student visas have had to pass strict vetting procedures introduced in September 2007.
The procedures include fingerprint tests and a check of applicants' identities against criminal and counter-terrorism databases, as well as additional immigration and asylum checks.
From autumn this year, British universities will also be obliged to check the names of overseas students against a government database of terror suspects.
Pictures of Manchester's Arndale Centre are thought to have been found
However, it is not yet known whether any of the suspects arrested on Wednesday applied for their overseas student visas after September 2007.
If they did apply after that date, it could mean that they passed all the tests designed to weed out extremists.
Immigration minister Phil Woolas told the BBC that secret British intelligence and intelligence from other countries was also being used "in a targeted way".
And he rejected criticism from Pakistan's High Commissioner, Wajid Shamsul Hasan, who accused the UK of refusing Pakistani offers of help with background checks.
"It's naive to think that we don't check, we do work very closely with the Pakistan authorities, indeed we've been criticised for doing so," Mr Woolas said.
The joint anti-terror approach was being reinforced by Downing Street on Friday, which said Gordon Brown and Pakistan's president were "committed to working together".
After Wednesday's raids Mr Brown challenged Pakistan to do more to weed out potential extremists who might target the UK.
A spokesman confirmed Mr Brown and President Asif Ali Zardari had since spoken on the telephone and agreed "the UK and Pakistan share a serious threat from terrorism and violent extremism".
Police are continuing to search properties in the north west of England
In Manchester, it was business as usual for the shopping centres allegedly named in the suspected plot.
Security staff at the Trafford Centre and officials at Manchester Arndale said they had not been informed of any threat.
And an Arndale spokesman said: "Both Manchester Arndale and the The Birdcage will be operating as normal over the Easter weekend."
Police are not thought to have recovered any explosive devices during their searches.
BBC security correspondent Frank Gardner said his understanding was that the alleged plot had been at the "aspirational, not operational" stage.
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