Foreign students are obtaining visas through language schools which offer little education despite Government attempts to crack down on such bogus colleges.
A Home Office task force set up last year has visited 1,200 independent "colleges" and found a quarter of them to be bogus. Hundreds were merely fronts to help immigrants apply for visas.
A register of providers now kept by the Department for Education and Skills is supposed to identify genuine colleges for which non-EU citizens can obtain student visas.
Since January this year, the colleges have been required to inform the Home Office of any students who do not attend classes regularly.
However, foreign students and college principals have told that there is still widespread abuse of the system, with back-street colleges happy to turn a blind eye to students' poor attendance records.
The Home Office issued student visas to 294,000 non-EU citizens last year.
The British Council and English UK, the largest language teaching association, run an accreditation scheme with 350 independent colleges on its books, but there are at least twice as many non-accredited colleges.
A Colombian man, who entered Britain on a student visa and asked not to be named, said: "The whole system is a farce. London is full of schools that will just accept your money so you can get a visa.
Foreign students studying for short (non-degree) courses can stay for a maximum of two years. They initially receive a one-year visa and must produce a satisfactory attendance record to extend their stay for a second year.
Many colleges make ambitious or misleading promises about the level that students can reach, or claim to be accredited by non-existent bodies.
A Home Office spokesman said: "We set up a task force in April last year which has visited 1,200 colleges and 25 per cent were found not to be genuine.
"Every college on the list has now been visited, but we are aware that new ones are springing up all the time and we are visiting them on an ongoing basis," Daily Telegraph quoted him as saying.