The September 2014 new car market was the biggest for the month since 2004, ending with 425,861 registrations.
It marked the 31st consecutive month of growth, although the increase slowed to 5.6% ahead of September 2013.
Registrations for the year-to-date have reached 1,958,196, up 9.1% on the first nine months of 2013.
After March’s 18% surge, September’s more moderate growth indicates demand is starting to level off, said the Society of Motor Manufacturers and Traders.
Mike Hawes, SMMT chief executive, said: "September’s strong performance underlined the continuing robustness of the UK new car market, particularly in the context of last September’s bumper volumes.
"Demand for the new 64-plate has been boosted by intensifying confidence in the UK economy, with consumers attracted by a wide range of exciting, increasingly fuel-efficient, new cars. In the months since March – which saw an 18% jump in registrations – the growth has shown signs of levelling off as the market starts to find its natural running rate."
Chris Sutton, managing director of motor finance provider, Black Horse said: “It is no surprise that we have witnessed a rise in car sales this month given it includes the new September plate change. However, we would anticipate the rate of growth in car sales to level out to more moderate levels.
“Whilst manufacturers and dealers alike have been working very hard to offer competitively priced finance deals to entice customers into the showroom they’ll now be turning their attention to consider how they will retain this custom and continue to make a profit.
"What is certain though is that as long as interest rates remain low and manufacturers offer affordable finance deals alongside technologically advanced cars the motor industry will continue to produce good news for the UK economy. All in all we have a lot to be proud of in how the industry has performed since the economic recovery has begun especially compared to our European counterparts.
TrustFord (formerly Ford Retail) operations' director John Leeman said: “We always see a healthy demand for the new plate in September and the new 64 registration would certainly be a key factor in why the sector recorded its 31st month of continuous growth.
"Such is the demand, a number of our dealerships across the UK stayed open late on August 31, so some of our customers could pick up their cars at midnight and be amongst the first drivers in the UK to sport the new plate.
“This hugely impressive run, which has been driven primarily by increased consumer confidence, was also mirrored at TrustFord. During the last three months we have seen a 15% increase in retail sales volume on the same period last year.”
Richard Lowe, head of retail and wholesale at Barclays, said: "The heady cocktail of a new plate change, great finance deals and the appeal of increasingly fuel efficient cars resulted in September being another strong month for new car sales.
"Consumers are also benefiting from stable residual values enabling them to trade in their old cars for shiny new models at a similar monthly cost. While the market is likely to balance out over the final few months of the year, 2014 is destined to be another success story for dealers and manufacturers.”
Models with the highest registrations in September:
1 Ford Fiesta 23,266
2 Ford Focus 13,508
3 Volkswagen Golf 13,011
4 Vauxhall Corsa 12,506
5 Volkswagen Polo 10,312
6 Fiat 500 9,122
7 Vauxhall Astra 8,861
8 Audi A3 7,317
9 Peugeot 208 6,968
10 Nissan Qashqai 6,847
Quarterly increases in new car registrations, Q1 2013 to date
For more SMMT data click here.