New car sales are down for the fifth month in a row
New UK car registrations fell 21% in September, the Society of Motor Manufacturers and Traders (SMMT) has announced.
This is the fifth consecutive month that the SMMT has reported falls in new car sales.
In further evidence that the current financial crisis is hitting the wider economy, year-to-date registrations are down by 7.5%.
The SMMT is urging the government to take action to restore confidence.
The figures are particularly disappointing given that September is typically a bumper month for new car sales - it usually accounts for 17% of the annual total.
September is second only to March - the other month in which car registration letters change - as the most popular month for buying a new car.
New car registrations have now fallen in all but two months of 2008. Since April, the 12-month rolling total has dropped by almost 150,000, with September accounting for 60% of this fall.
Speaking to the BBC, Paul Everitt, chief executive of the SMMT, said: "This represents the worst decline in new car and commercial vehicle registrations since 1991."
New business registrations were hit particularly hard, falling 37%. New private registrations were down 23% and new fleet registrations were down 16%.
The SMMT believes that the market may not stabilise before the third quarter next year.
Small cars, however - the so-called mini segment - bucked the trend and posted growth in the month, thanks largely to Smart, Hyundai's i10 and Vauxhall's Agila models.
Large cars generally saw sharper falls than small cars, although both Audi and Jaguar saw sales increase.
The Vauxhall Corsa was the best selling car in September, but the Ford Focus remains the UK's best seller this year.
The SMMT is concerned that the government is too preoccupied with the financial crisis. Mr Everitt said: "We're hearing a lot about what's happening with the banks but there are concerns that these real economy impacts are not really registering."