UK new car sales rose 13.5% in May compared with the same month a year ago, according to the latest industry figures.
The number of new cars sold in the UK during the month was 153,095 compared with 134,858 in May last year, the Society of Motor Manufacturers and Traders (SMMT) said.
The increase marked the 11th successive monthly rise.
But the SMMT warned the coming months would be "extremely challenging".
It added that sales for the full year were expected to "dip slightly" after the UK's scrappage scheme closed.
The scheme, which offered new car buyers a £2,000 discount if they scrapped a car that was more than 10 years old, started on 18 May 2009 and came to an end on 31 March 2010.
The SMMT said almost all scrappage orders had now been delivered, with only 2.7% of sales in May coming through the scheme. That compares with an average of 17% a month since the scheme started.
"It is essential that the upcoming emergency Budget promotes consumer and business confidence to maintain economic recovery," said Paul Everitt, SMMT chief executive.
The Ford Focus was the best-selling car in May, knocking the Ford Fiesta off the top spot for the first time this year. The Fiesta came in third after the Vauxhall Astra.
Sales to private buyers rose 12.3%, while sales in the fleet market increased 16%.
Registrations of alternatively-fuelled vehicles rose 195% to take 1.1% of May's market share. A year ago, they made up just 0.4% of the market.
Sales volumes over the first five months of the year had risen 22% from a year ago, the SMMT said, reflecting the weakness of the 2009 market.