UK new car registrations appeared on course for their first annual fall since 2011 after a 9.3% tumble in September, a month which normally accounts for around 15% of demand.
Sales were hurt by business and political uncertainty as Britain prepares to leave the EU and confusion over government plans on diesel and petrol cars.
This could include new levies or restrictions, the Society of Motor Manufacturers and Traders (SMMT) said.
UK sales totaled 426,170 vehicles with demand falling across the board, SMMT data showed, leaving year-to-date registrations down 3.9%.
Demand has fallen year on year since April due to acombination of factors including an increased vehicle excise duty, weaker consumer confidence and record sales in 2015 and 2016.
But September is normally a strong month for car sales, partly due to the fact licence plates indicating the age of a vehicle change on March 1 and September 1.
Demand for diesel cars in the UK slumped 21.7% and petrol fell 1.2%. Sales to fleet business buyers declined 10.1% and to consumers fell by 8.8%.
Only demand for alternative fuel vehicles, namely electric and hybrid plug-in vehicles, rose, increasing by 41% - albeit from a very low base.
The depth of the slump in September comes despite almost all major UK car companies offering thousands of pounds off new models with scrappage schemes, trade-in programmes and discounts.