Political and economic uncertainty, not least from Brexit, sent sales of new cars in Britain falling sharply in September, a key month for the industry, data showed on Thursday.

New car sales fell by 9.3 percent year-on-year to 426,170 vehicles in September, industry body, the Society of Motor Manufacturers and Traders (SMMT) said in a statement.

It is the first time in six years that new registrations fell in September, a month in which a change in number plates usually buoys the automobile industry. But this has not been the case this year amid fallout from Britain's decision to quit the European Union. 

"September is always a barometer of the health of the UK new car market, so this decline will cause considerable concern," said SMMT head Mike Hawes. 

"Business and political uncertainty is reducing buyer confidence, with consumers and businesses more likely to delay big-ticket purchases," he said. 

Confusion around government air quality plans may also have contributed to the decline, Hawes continued. 

Under a change in UK tax rules that came into effect in April, all new cars -- except for those with zero carbon dixoide emissions -- are now subject to an annual flat-rate charge. 

Previously, low-emission models had also been exempt.

Demand for petrol-fuelled cars fell 1.2 percent in September, while diesel tumbled 21.7 percent. 

Sales of alternatively fuelled vehicle, including electric and hybrid cars, surged 41 percent, the industry data showed.

Looking at the nine months to September, total new car sales were down 3.9 percent from the year-earlier period. 

The best-selling car in Britain last month was the Nissan Qashqai, followed by the Volkswagen Golf, with the Ford Fiesta in third place.