More than 400,000 new cars were sold in September, a huge increase on a year ago and the biggest monthly total for more than five years.
The September 63 plate boosted sales, which were 12.1% higher than the same time last year, to hit 403,136 registrations , the Society of Motor Manufacturers and Traders (SMMT) said .
More new cars were sold than in any month since March 2008.
Private sales have increased by 16.7% over the year to date.
More than one in seven new cars registered in September was built in the UK.
Mike Hawes, SMMT chief executive, said: "With over 400,000 new cars registered for the first time in more than five years, the UK market is reflecting growing economic confidence.
"Robust private demand has played a major role in this growth with customers attracted by exciting increasingly fuel-efficient new models that offer savings in the cost of ownership.
"This is the 19th consecutive month of steady growth and, with fleet and business demand still to reach pre-recession levels, we believe the performance to be sustainable.
"The latest 63-plate should deliver positive results into next year."
The Ford Fiesta was the best-selling model last month.
Ford said its sales topped 51,000 last month, up by more than 3,000 on a year ago.
The Fiesta and Focus are the two best-selling models in the year to date, with both recording rises in sales and their share of the market.
Land Rover said it had its best ever September, with sales of 10,769, up by 11% on a year ago.
Jaguar saw a 19% increase in the year to date.