Registrations were down 11.5 per cent compared with same month last year, according to the Society of Motor Manufacturers and Traders (SMMT).
The government's Scrappage Incentive Scheme helped to lift sales last year.
The programme expired in May and sales have been falling since July.
SMMT chief executive Paul Everitt said: "Registrations are expected to fall next month, but demand may benefit from motorists looking to avoid the January VAT rise.
"Next year will continue to be challenging as consumer spending tightens and the government's austerity measures take effect."
Diesel car sales continue to overtake those of petrol cars and now account for 53 per cent of the market.
Meanwhile fleet car sales rose six per cent – as companies sought to beat the looming rise in VAT.