Cgu admits insurance from its affinity partnership with Vauxhall motors is generally cheaper than that supplied to its brokers (see letters page six).

The two-tier price structure was discovered by North West broker Alan Tees when he went to buy a car from a Vauxhall used car dealership.

He was amazed to find CGU was able to offer two years' comprehensive insurance to 17-year-old motorists for £320 under a "free insurance" deal.

Tees said: "CGU will not even provide a quote for us for a 17-year-old, and the cheapest we can access is Zurich at £3,309."

A CGU spokesman confirmed it was possible for a 17-year-old driver to obtain cheaper insurance under a short-term promotion with its affinity partner Network Q, Vauxhall's secondhand car dealership. However, he stressed that premiums entirely reflected the age and geographical profile of the business and could rise as well as fall.