A used car pricing realignment is expected at the end of this year, caused by high and increasing volumes of stock and reduction in values.
The warning comes from CAP after record September new car sales and a summer that exceeded market expectations.
Derren Martin (pictured), senior editor of CAP Black Book, said: “September will undoubtedly see large numbers of retail part-exchanges and fleet returns.
"Early September may well stay relatively steady with regards to value movements, with particular pressure not appearing until the additional volumes start to appear.
“With used car volumes being high, and on the increase, and reductions in values so far this year being relatively modest, we are faced with a similar picture to that of last year.
"In the final three months of 2014 there was a pricing realignment in the used car market and there is every likelihood that this could happen again, although this time around there is the potential for it to start earlier and still last for the remainder of the calendar year.”
Used car supply levels were high in August, but closely matched by demand.
Values in Black Book Live declined slightly, with average value drops a 0.6% at the three-year/60,000-mile point.
City cars and convertibles saw the heaviest reductions in percentage terms.
City cars suffered from high volumes, CAP says, and new car price pressure, with the Renault Twingo, Smart Fortwo Diesel, Proton Savvy and Vauxhall Agila, the hardest hit.
Convertibles saw pressure from the usual seasonal trends, the BMW 1 Series, current Audi S5, and the Smart Fortwo Diesel saw some of the larger percentage drops.
As the end of the summer approaches, trade buyers are reluctant to buy convertibles for stock, with consumer demand tailing off rapidly.
CAP has reported a positive response from the market to its “little and often” movements that reflect changes as they happen.
This is seen infinitely more accurate than large monthly value drops.