Unlicensed minicabs are being used to drive children around London due to a loophole in the law, a union warns.
The GMB union says taxi firms that have school contracts are avoiding licensing by saying that their vehicles are not made available to the general public.
This means they can sidestep rules that ensure minicabs and cab firms are licensed and their cars are roadworthy.
Mayor Ken Livingstone and the GMB have now written to all councils warning of the dangers of using unlicensed firms.
Under the Private Hire Vehicles Act of 1998 vehicles must be subjected to extensive checks in order to legally carry private passengers.
Terrence Fletcher, of the GMB's professional drivers' branch, said: "They (drivers) could have already been refused a licence from the Public Carriage Office (PCO) because of health reasons and these companies are at liberty to use them."
Mr Fletcher explained that licensed vehicles undergo two MOTs a year and a PCO inspection.
"These vehicles won't have been tested in any way" and will not have certain insurance documents which are a requirement for licensed cabs," he added.