The warning from BCA is the latest in a series of alerts over the importance of the registration document in selling vehicles from next year.
From February 1, car owners will have to present a V5 document in order to obtain a vehicle tax disc. It is vital for the appropriate part of the registration document to be passed on to the buyer when the vehicle is sent for auction or sale.
BCA is working with fleets, dealerships and leasing companies to emphasise the growing importance of the document.
Tom Madden, director, customer affairs for BCA, said: 'If you don't have the V5 at sale time then you are in trouble. But it is surprising that despite the importance of the changes, few people seem to be talking about it.'
In October, Manheim Auctions senior group auctioneer Andrew Shepherd warned that the issue was a major concern for the entire industry.
He was backed by Martin Ward, national research manager for CAP, who said the need to include a V5 would dramatically change what the trade would buy at disposal time.