The extent to which bogus colleges are being used to help illegal immigrants enter Britain was exposed on Tuesday.
Almost half of private colleges visited by inspectors have been struck off an official list of approved providers.
Some were removed for “technical” reasons, but many others are understood to be fronts for student visa scams.
Out of 256 colleges checked since the register was set up three years ago, 124 were removed.
But with as many as 1,750 private colleges still to be inspected, more could yet be exposed.
Immigrants pay hundreds or even thousands of pounds to the fake college, where no classes ever take place, to become “students” and qualify for temporary visas.
It is cheaper and safer route into Britain than paying to be smuggled in by organised gangs.
When the visas expire, the immigrants apply for an extension or simply “disappear” from records by remaining in Britain illegally.
The revelation that a large number of colleges have been caught in the crackdown on bogus institutions follows the Daily Mail’s disclosure that immigration officers have been ordered to stop deporting foreign students who overstay their visas.
A leaked memo suggested they are not regarded as a high enough priority by the Home Office.
The Department for Innovation, Universities and Skills said there were 2,000 private colleges on the register of approved providers.
Students who gain a place at one of these institutions are eligible for a visa.
But there are concerns that many of the remaining 1,750 colleges on the list have not yet been checked.
An investigation by BBC London News accused a bogus university of trading off the reputations of Oxford and Cambridge to fool foreign students into paying out thousands of pounds for worthless degrees.
It revealed how the “college” has staged degree ceremonies at the institutions by hiring out university venues including Cambridge’s oldest college, Peterhouse.
The Irish International University has been affiliated to at least one backstreet London college which offers its dubious qualifications.
Foreign students had been granted visas to study there since it appears on an official Government register of education providers.
The university said it has undergone a radical overhaul and that it will not renew its affiliations with any private colleges.
From 2009, colleges must be accredited with an approved independent body to be included on the register.
Tory universities spokesman David Willetts said: “It is very worrying that so many of the colleges on the Government’s list have turned out to be bogus.
“It begs the question of how they got on to the list in the first place and suggests the Government’s process for accrediting them is not up to scratch.”
Higher Education Minister Bill Rammell said: “The vast majority of private colleges in London operate lawfully and provide a high-quality service to their students.
“Since 1 January 2005, 256 colleges on the DIUS Register of Education and Training Providers have been investigated by the Borders and Immigration Agency.
“Of these, 124 have been found to be in breach of the Immigration Rules, and therefore removed from the register.
“We are working very hard on behalf of students to ensure that all private institutions meet strict quality standards.
“Where we are not satisfied that this is the case with a particular college, we will not hesitate to investigate and if necessary, close it down.”