David Hill, senior editor of CAP Red Book, made the warning as Chancellor of the Exchequer Gordon Brown prepares to give his pre-Budget statement on Wednesday.
Double-cab vehicles with a payload of more than one tonne are currently classed as commercial vehicles and qualify for a maximum benefit-in-kind tax liability of £500 per year, making them an attractive alternative for company car drivers looking to minimise their tax bills.
Hill said: 'Good news or not, what we must watch out for are changes in the rules slipping in with little notice because that is often the first step in a journey towards trouble with the Inland Revenue.
'There are people who have been running around in £20,000 vehicles for £500 a year in tax liability and a tax rule change would bring a flood of such vehicles into the marketplace. This would result in plummeting values and a major upset for a market that has been very buoyant.'