Some UK language schools are being used as bases by people selling false papers enabling immigrants to obtain student visas, the BBC has found.
Some are paying up to £600 for fake documents which they use in visa applications to extend their stay in the UK, the Ten O'clock News reported.
It filmed people selling documents to immigrants who had indicated they had no intention of studying in the UK.
The Home Office plans a more rigorous system of registering colleges.
It believes more than half of 83 registered language colleges it inspected recently should be shut down.
The Home Office will grant a student visa only if the applicant is studying at a registered school.
Officials said a register of education providers was introduced last January.
And in the future it will be made mandatory for colleges to make it compulsory to report students who do not attend classes regularly.
But the Association of Colleges - which represents the mainstream further education sector - said it had repeatedly warned the Home Office throughout last year that its failure to tighten the rules for inclusion on its official Register of Language Schools would allow many such colleges to abuse the system.
The BBC investigation found cases of people paying £300 to £600 for documents, such as false attendance records, suggesting they were studying at certain colleges.
It is estimated that up to 50,000 students could be using such scams to stay in the UK illegally.
One of the language schools involved in the BBC investigation said it would investigate the allegations and discipline anyone found to be in contravention of its rules.
In one highlighted case, £350 was paid for papers saying an applicant had completed an eight-week course despite her making it clear she had no intention of studying.
A man connected with a different centre offered to arrange a visa for £300 without the need to attend the school.
Scratching the surface?
In another example, a man came to the UK on a student visa and extended it twice without going to any classes and is now working in the building trade.
"When my visa expired, I found out from some friends of mine that you could get it extended," he told the BBC.
"So I paid £600 for a one-year college course."
Industry body English UK believes around 10% of registered centres may be breaking the rules.
Immigration officials have told the BBC they are determined to crack down on rogue elements connected to language schools.
The Conservatives have called for urgent action to be taken.
They said the findings highlighted the crisis within the immigration service and suggested they could merely have scratched the surface of the problem.