September’s slump was caused by a variety of factors including changes to the way new vehicles are tested, with tougher emissions regulations ushered in by the European Union.
Trade group the Society of Motor Manufacturers and Traders (SMMT) said that just under 339,000 new cars were registered across the UK during what is normally one of the industry’s strongest months due to the release of new number plates. It marks a fall of 20.5 per cent compared with September 2017.
Diesel and petrol registrations were down year-on-year, with a modest rise for alternatively fuelled cars such as pure electrics and hybrids.
SMMT chief executive Mike Hawes said: “This year we’ve had the first major change in the way you test vehicles – the first change in about 30 years.
“That has led to some major challenges in terms of supply because you’ve got to change the entire European model range, put them through the test, and that takes a considerable amount of time.
“Some manufacturers have had short supply which has meant sales have been down.”
He added that “demand is down a bit” due to a drop in business and consumer confidence amid uncertainty over Brexit.
Alex Buttle, director at car buying comparison website Motorway.co.uk, said: “These figures will send shock waves through the industry.
“Even taking into account regulatory changes and more registrations bumped forward to August (as car makers rushed to register vehicles in bulk to beat the new European emissions testing deadline), a 20 per cent drop on last year’s numbers is astonishing.
“We are now entering a crucial and unprecedented period for the car industry, as the next new number plate will be March 2019 when the UK is due to leave the EU.
“It’s likely to be a rollercoaster ride for new car sales figures for the foreseeable future, but it feels like we have just plunged into a deep canyon.”
Sue Robinson, director of the National Franchised Dealers Association, said: “The introduction of the Worldwide Harmonised Light Vehicle Test Procedure (WLTP) in September has affected not only the UK but also other major European markets which, in many cases, fared worse.
“The declines across the main European markets demonstrate the challenges that WLTP has posed to manufacturers not only in the UK.
“Consumer footfall has remained fairly robust although a proportion of motorists feel unsure about which car to buy due to the lack of clarity surrounding a number of key policy areas.”
The UK-wide figures showed that the Ford Fiesta remained the best selling car last month, followed by the Vauxhall Corsa.