GISC has received over 400 complaints against non-standard motor broker Allen & Allen.
The broker, which picked up a major industry award for its broker services last week, is thought to have attracted complaints centring on administration, documentation and pricing.
An industry insider said: "Their reputation on administration is dreadful. What they are going to do in the new FSA environment is anyone's guess."
Because Allen & Allen is not a member of GISC, a spokeswoman for the organisation would not comment about the complaints.
But she did say: "As non-members there is no independent arbitration body for customers to appeal to."
Tony Allen of Allen & Allen responded to the allegations by saying: "We are a big organisation with over 300 staff so there are always going to be some complaints, but compared to our size they are minimal."