More than 4.18 million used vehicles were sold in the first half of 2016, a rise of 8% on the same period last year.
This compares to a 3% increase in the sales of new cars seen in the first half of the year.
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While the rise shows the second-hand market is booming, the Society of Motor Manufacturers and Traders (SMMT) said future growth could be dependent on a number of factors.
The SMMT’s chief executive Mike Hawes said: “The UK's used car market is at its strongest ever.
“The growth in the used car market has reflected the record demand for new cars in recent years, but future growth in high-cost purchases will depend on stable consumer and business confidence.”
The figures, which have been put together by the SMMT, also show that the second-hand car market has made gains on its position a few years ago.
In the wake of the 2008 financial crisis, sales of used cars plummeted before reaching a standstill.
In 2009, the Government was forced to introduce a scrappage scheme to encourage drivers to buy new cars.
Under the scheme, the Government agreed to contribute to the cost of new car when drivers scrapped their old one.
Levels of sales have been gradually rising since this low point, with popular vehicles including those that are between one and three years old, super-minis and small family cars.
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Sales have grown in earnest over the last three years, going on to hit a peak in 2015, the figures show.
Financial observers use car sales figures as an indicator of consumers’ willingness to spend money, as well as a bellwether for the strength of the wider economy.
In July, the boom in sales of new cars seen last year appeared to slow down slightly, separate figures from the SMMT suggested.
Sales grew by just 0.1% last month after record levels of growth in 2015 and a strong performance in the early part of the year.