Bogus students are misusing Britain's points-based immigration regime to slip into the country, according to an official document.
An immigration intelligence unit said the student visa system was allowing numerous illegal immigrants to arrive and officers at ports of entry were powerless to stop them, according to a leaked UK Home Office memo.
It warned that it had "presented a window of opportunity to both individuals and organisations seeing easy access to the UK".
"The system is significantly weaker and allows a large number of bogus students to gain entry to the UK", The Daily Telegraph newspaper quoted the memo as saying.
The note focused on abuse of "Tier 4" of the points-based system, which covered student visas and was introduced last March.
A group of Pakistani men suspected of a plot to blow up shopping centres in Manchester last Easter had arrived in Britain on student visas.
They were later released without charge but made subject to deportation on national security grounds.
Bogus students who were refused or would have been rejected under the old regime were being waved through even though border control staff were convinced that they were not genuine, the report in the British daily said.
About 1.5 million student visas have been issued in the past eight years, including 236,470 in 2008-09, but the memo to the Home Office from the Heathrow Intelligence Unit claimed that it was education institutions that were effectively deciding who was allowed to enter.
Port officers are required to assess students in a completely different way to the old regime. It is largely perceived across Border Force that PBS is providing entry clearances to students that would have been refused in the previous system," the Home Office memo said.
"One officer reported that a recent Bolivian passenger was refused a student visa under the old policy but was successful under the new with no differences between the two applications," it noted.
Under the points-based system, education institutions must be approved by the Home Office to act as sponsors to bring in foreign students and it is they that are responsible for assessing whether someone is eligible to enter.
The memo warned that unless the officers could provide that documents were false they could not refuse entry and described the new system as a "tick box" process that "removed the ability of the entry clearance officer to assess
credibility of either the college or the applicant."
Phil Woolas, the Immigration Minister insisted that the points-based system was a more "transparent, objective and
robust application system than what went before."
He said the Home Office had acted on concerns raised by staff and that Prime Minister Gordon Brown would soon receive a review into Tier 4 that would include changes to make it even more robust.