The UK car market shrunk by 9.3% to 426,170 units last month, marking the first September in which registrations declined for six years.
Sales usually grow in September due to the registration plate change.
Society of Motor Manufacturers & Traders (SMMT) CEO Mike Hawes said: “September is always a barometer of the health of the UK new car market, so this decline will cause considerable concern.
“Business and political uncertainty are reducing buyer confidence, with consumers and businesses more likely to delay big-ticket purchases.”
Hawes labelled “confusion over air quality plans” as one of the key reasons for a large decline in demand for diesel cars, registrations of which are down by 21.7% compared with September 2016. There was also a 1.2% decrease for petrol models, with 232,810 cars registered.
Contrastingly, registrations of alternatively fuelled vehicles (AFVs) continued to soar, with a 41% boost in output over last year’s month. Despite the boost, however, AFVs still accounted for just 22,628 cars, or 5.3% of the market, in September.