Last month was the slowest September for new car registrations since 2000.
Sales were down 3.2% on the same time last year, with just 417,052 cars leaving forecourts.
These latest figures from the Society of Motor Manufacturers and Traders (SMMT) reflect a slower year-to-date, with overall sales slipping 5.2% in the first nine months.
Much of this can be attributed to a large drop in the number of private sales. Trends suggest that new car buyers are becoming more cautious, possibly in response to rising fuel prices.
Sales of diesels climbed for the 60th consecutive month, accounting for 37% of September's market. The Ford Focus was the best-selling diesel model last month, as well as the best-selling 55-plate car overall. Ford shifted nearly 5000 more Focuses than the second-placed Vauxhall Corsa.
SMMT chief executive Christopher Macgowan said: 'The new car market is fiercely competitive this year and manufacturers are fighting hard for every sale, particularly for private customers.
'However, while overall registrations are down, diesel has continued what seems an inexorable rise in popularity. That may well have a lot to do with soaring fuel costs and the economy benefits that diesel brings to drivers.'
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