Almost half the colleges checked on an official list of approved providers for overseas students have been struck off, the government has said.
Following fears about bogus colleges, the government said it had inspected 256 colleges since 2005, leading to 124 being removed from the list.
There are about 2,000 private colleges on the Department for Innovation, Universities and Skills' register.
A place at one of these colleges is a means to getting a UK student visa.
A Dius spokesman says that colleges on the register of education and training providers are now facing unannounced inspections.
But Conservative university spokesman, David Willetts, expressed concern that so many colleges on the government's approved 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 governments process for accrediting them is not up to scratch," said Mr Willetts.
A BBC investigation this week revealed an example of a college offering courses with no legitimate accreditation - described as "dodgy" by its own chancellor.
There has been a longstanding problem with bogus colleges - often either set up to sell fake degrees or else as part of an immigration and visa scam, allowing people to enter the UK as students.
The approved register of education providers was created in 2005 to prevent such abuses - requiring applicants for student visas to show that they would be attending a legitimate institution.
But this register, the gatekeeper for those seeking student visas, is now under scrutiny - with an unspecified number of bogus colleges among those removed from the list.
Dius says that the check ups, aimed at preventing bogus colleges, might also have found other technical reasons for removing the registration of institutions.
It also remains unclear how many of the remaining 1,750 colleges have ever been physically inspected before or after inclusion on this register.
Among those currently on the list is a college whose website content is mostly links to services including online gambling.
Bogus colleges charge overseas students thousands of pounds in fees for a place - which is then used as a way of entering or remaining in the UK on a student visa.
Such bogus colleges often claim to have well-equipped campuses, but in reality are operating from temporary office addresses. Websites can have pictures of college buildings or landscapes which are nothing like their authentic locations.
Overseas students are required to attend courses for at least 15 hours per week. But they are allowed to work - and their spouses and children are also entitled to work in the UK.
A Dius spokesperson says that the integrity of the list is taken very seriously - and that the vast majority of private colleges are legitimate.
The introduction of unannounced inspections in November was intended to catch bogus colleges which might not really have the students, facilities and staff claimed when they registered.
But the list does not make any assurances about the quality of the education available at these institutions.