Crawley Borough Council received 27 complaints about mini-cab companies since October.
A council spokesman said it received ten email complaints in October, 14 in November and three last month.
Cllr Michael Jones, chairman of the Licensing Committee, said numerous companies had been reported to the council.
He said: “Whilst it is not possible to comment further at this time due to the ongoing nature of the investigation the public are advised to be vigilant when using local private hire companies in cases where a vehicle not licensed by Crawley Borough Council arrives to fulfil their booking.
“In such cases it is possible that the driver has not been vetted to the standards operated by Crawley Borough Council and that the vehicles insurance will not be valid.”
The council requires drivers to pass a test of their knowledge of Crawley roads to obtain a private hire or taxi license.
Cllr Jones (Lab, Bewbush) said: “The council has stringent standards and these are applied to all new drivers.
“The council will not confirm the identity of any private hire operator before it is fully satisfied any complaints made against them are justified.
“Any complaints the council receives relating to taxis are actively investigated.”
Passengers were advised to pre-book licensed private hire vehicles with the company and check their rear plates have the council’s name.