Latest Society of Motor Manufacturers and Traders (SMMT) new car registration figures reveal the best-selling car of 2010 so far sold less models in May than the larger Ford Focus.
It was also surpassed by the Vauxhall Astra in May as sales of city cars and small superminis dipped.
Ford sold 6,619 Fiestas in May, over 100 less than total sales for the Vauxhall Astra. The Focus was easily the best-selling car in the UK with 7,163 units sold.
However, the Fiesta remains the most popular new car in the UK in 2010 with nearly 50,000 sales so far, 9,000 ahead of the Focus in second place.
Vauxhall models also continue to perform well, with the Astra and Corsa selling 36,099 and 35,162 respectively so far this year.
The Peugeot 207 and Renault Clio city cars retain their positions in the top ten with 20,345 and 17,454 sales despite not coming in the top ten best sellers in May.
The SMMT figures show new car registrations rose by 13.5 per cent in May compared to the same month in 2009 despite warnings of an imminent slump in sales from industry experts.
"May was another good month for the UK new car market, although we expect the coming months to be extremely challenging," said Paul Everitt, SMMT chief executive. "It is essential that the upcoming emergency budget promotes consumer and business confidence to maintain economic recovery."
The SMMT warns the increase year-on-year is largely down to poor performance in 2009 at the height of the recession. Sales in May were down by 14.6 per cent on 2008 levels – the equivalent of 26,177 units.
Final scrappage scheme registrations have fed the market in May and several manufacturers have cut prices, refreshed their model ranges and offered discounts to boost sales.