There was a 7.9 per cent growth in the used car market over the same period in 2015, with more than 4 million vehicles changing hands.
Figures from the Society of Motor Manufacturers and Traders (SMMT) showed that 4.18 million second-hand vehicles were sold in the first six months of the year - the first time such sales have exceeded four million.
The rise in used car sales accompanied a continued growth in new car sales, although deals on brand new models only rose by three per cent over the same period.
Mike Hawes, chief executive of the SMMT, said: "The UK’s used car market is at its strongest ever, and that’s great news for buyers spoilt for choice with the latest advanced vehicle technology and attractive financing options."
As well as being the UK’s best-selling new car the Ford Fiesta topped the table in used car sales, with 60,000 models being sold on.
In fact, small family cars (27.2 per cent) and superminis (33 per cent) accounted for almost two thirds of all used car transactions.
April saw the largest number of sales, reflecting the registration plate change at the end of March, and models between one and three years old were the preferred purchase, according to the SMMT figures.
While the latest figures mirror growth seen in 2015, Mr Hawes warned that continued expansion of the market was not guaranteed.
"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," he said.