Data shows that 153,224 cars rolled of production lines last month compared to 159,726 in September 2016 (-4.1%).
Domestic demand fell from 36,607 cars in September 2016 to 31,421 cars in September 2017 (-14.2%).
Meanwhile, export demand fell from 123,119 in September last year to 121,803 last month (-1.1%).
Year-to-date data shows that show far, in 2017, 1,259,509 cars have rolled off production lines compared to 1,287,680 last year (-2.2%).
SMMT chief Mike Hawes says: “With UK car manufacturing falling for a fifth month this year, it’s clear that declining consumer and business confidence is affecting domestic demand and hence production volumes. Uncertainty regarding the national air quality plans also didn’t help the domestic market for diesel cars, despite the fact that these new vehicles will face no extra charges or restrictions across the UK.
“Brexit is the greatest challenge of our times and yet we still don’t have any clarity on what our future relationship with our biggest trading partner will look like, nor detail of the transitional deal being sought. Leaving the EU with no deal would be the worst outcome for our sector so we urge government to deliver on its commitments and safeguard the competitiveness of the industry.”