May's registration figures mark the longest ever period of sustained growth in the new car market.
Mike Hawes, Societry of Motor Manufacturers and Traders (SMMT) chief executive, said, "The new car market has now grown in every month since March 2012 - the longest period of growth on record and a reflection of the UK's ever-improving economic conditions.
"Over the past 27 months, increasingly confident consumers have been drawn to some fantastic new products, attracted by innovative technologies, improved fuel economy and competitive deals helping make a new car more affordable.
"With SMMT forecasting an overall rise of around 6% over the year, the coming months should see some levelling off in growth rates as underlying demand stabilises."
According to SMMT figures, fleet registrations took a 49.3% share of May's market, down from 49.9% in May 2013, while registrations to small businesses increased taking a 3.9% share, compared to 3.3% a year ago. Retail was stady on 46.8%.
Diesel cars increased their share of the new car market in May to 51.3%, up from 50.8% a year ago, while alternative fuelled vehicles, which includes hybrids and plug-in cars, took a 1.8% share of the market, up from 1.1% in May 2013.
The Ford Fiesta was the best-selling car in May, with the Volkswagen Golf in second place.