Transport for London has announced an "urgent review" after the BBC found thousands of London minicab drivers may be working fraudulently.
Drivers must sit mandatory exams at TfL centres or authorised private schools or colleges to get a licence.
A BBC undercover investigation exposed private colleges cheating the required tests.
TfL, the licensing authority, said it was investigating 3,700 certificates issued by private bodies.
It said this represented just three per cent of minicab drivers in Greater London who had been licensed by a private college under TfL exemption rules.
Deputy mayor for transport Heidi Alexander said: "We're doing an urgent review of every single minicab driver who's been issued a licence through this exemption route."
She said the review would be completed "by the end of next week".
The growth in taxi booking firms such as Uber has led to an 86 per cent surge in the number of private hire vehicle licences issued in the capital. Between 2011 and 2018, the amount has risen from 61,200 to 113,645.
Under the cab application process, along with a criminal record check and medical test, drivers must sit a topographical exam and an English test at one of eight official TfL testing centres.
Evidence of these exams can also be accepted via other qualifications, including BTecs, which are usually taken at private colleges and centres.
The qualification can be used to gain a minicab licence from many councils across the UK.
Employees at one of these colleges, Vista Training Solutions in Newham, east London, offered to take the tests for several BBC researchers for £500 per BTec.
Helen Chapman, TfL's director of licensing, regulation and charging, said: "We have put any applications on hold from those candidates who have not taken a topographical assessment in a TfL centre and are conducting an urgent investigation into drivers licensed with this exemption from Vista Training Solutions, based on the broadcast footage.
"We are working with the regulator Ofqual, who regulate the qualifications, and Pearson, the examining board, to be assured this fraud has only taken place at a single college.
"We are also reviewing whether to remove this exemption in its entirety."
Pearson said it was looking into the allegations and was "making the police aware of the situation".
Ofqual said it was "closely monitoring Pearson's investigation".
Vista Training Solutions said it was "devastated to learn that such malpractice took place" and apologised "unreservedly".
Anyone "concerned they are not properly qualified to drive a taxi" can retake the programme free of charge, it said.