It is demanding an immediate rewrite of legislation amid growing fears that poorly worded laws on private hire vehicles could affect other industries.
The problem has arisen because of the way laws such as the Private Hire Vehicles (London) Act 1998 and Public Passenger Vehicles Act 1981 are interpreted.
The first test of the 1998 Act comes next month, when licensing is introduced in London.
It defines a private hire vehicle as a 'vehicle constructed or adapted to seat fewer than nine passengers available with a driver to the public for hire and the purpose of carrying passengers other than a licensed taxi or public service vehicle.'
This could apply to a whole range of fleet vehicles, including green transport vehicles offering lifts to staff and courtesy vehicles used by garages to take customers home or to work while their car is in for a service.
The new legislation just affects London currently, but local authorities throughout the country are considering using the capital's programme as a blueprint for their own minicab licensing schemes.
When licensing is introduced in London on October 22, there is an application fee of £425, followed by a grant licence fee of £375. For running more than two vehicles, the fee jumps to £975.
RMI chief executive David Evans said: 'We are seeking clarification from the Department of Transport, Local Government and the Regions as to the extent of the regulations.
'If it becomes necessary, we will push for an amendment to be made to take courtesy vehicles out of the regulations altogether.'
A spokesman for the Public Carriage Office in London said although the legislation was not specific, it did not see problems arising for garages or green transport scheme operators.