The average value of used cars at the three-year, 60,000-mile point has fallen by 2.2% during July, according to Cap HPI.
The data firm says this is the biggest July drop it has ever recorded, using its real-time Live tracking product.
It is significantly more than the seasonal norm over the last five years, which was a 1% fall.
Cap HPI head of UK valuations Derren Martin said: “So far this year, every movement has been more negative than that of the same month in 2017 and 2018.
“The largest drops in Live in July were in the large executive and luxury executive sectors, with both falling out of favour with buyers, due to a reluctance to invest in big-ticket items.
“Drops have averaged over 3% for both sectors, which are significant amounts of around £800 for large executives and over £2,000 for luxury models.”
According to Cap HPI, the upper-medium sector saw a noticeably higher drop in the value of diesel cars over the last few weeks, said to be driven by volume and an abundance of similarly specified ex-fleet cars.
Electric cars are said to have moved back in line with the overall market, with more acute drops at the top end.
Martin said: “Diesel car values dropped at a slightly higher rate than petrol ones, as they have done during every month this year, but it is marginal.
“With clean air zones appearing there could well be some regional bias on pricing going forward and wholesale vendors would do well to be mindful of this when choosing where to sell, and we have already seen the early signs of this in our retail data.
“While there are some signs of recovery, the used car market is certainly not buoyant.”
