September figures were down 5.2% at 1,971,880 units. However, September marked the 60th month of diesel growth with a best ever total of 153,314 units.
Christopher Macgowan, SMMT chief executive, says: "The new car market is fiercely competitive this year and manufacturers are fighting hard for every sale, particularly to 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."
"If a slight depression has descended for some, following today's new car registration figures - which show a dip of 3.2% in September 2005 to 417,052 units, with year-to-September figures down 5.2% at 1,971,880 units - it can be explained to a large extent by the 'Rover' effect," said Sue Robinson, director of the Retail Motor Industry Federation (RMI) National Franchised Dealers Association.
"However, it must also be recognised that we have sold more cars in the first nine months of 2005 than in the whole of 1995 - only ten years ago," she said.