FRANKFURT (Reuters) - European car sales remained buoyant in June, except in Britain, where they fell for only the second time in four years amid uncertainty about the country's vote on EU membership.

Total registrations of new cars in the European Union and the European Free Trade Association increased 6.5 percent in June on the year, to 1,507,303 vehicles, data provided by the Brussels-based Association of European Carmakers (ACEA) showed on Friday.

That was much slower than a 16 percent increase in May, partly due to seasonal factors, but sales in Italy and Spain rose at double-digit rates last month from a year earlier and Germany, Europe's largest new car market, posted 8.3 percent growth despite smaller discounting.

Analysts cheered the increases but warned that sales momentum may not continue against a backdrop of economic uncertainty in the wake of Britain's decision to leave the European Union on June 23.

"While the European market remains buoyant, we continue to have reservations and see risk with respect to H2 and 2017 in the wake of the UK’s vote to leave the EU," analysts at Evercore ISI said on Friday.

Across Europe, sales of Volkswagen <VOWG_p.DE> brand cars slipped in June as the German carmaker continued to deal with the fallout from its emissions scandal, but luxury brands such as Mercedes <DAIGn.DE> and BMW <BMWG.DE> and makers of smaller cars including Fiat <FCHA.MI> and Renault <RENA.PA> saw solid sales.

In Italy and Spain total new car registrations rose 11.9 percent and 11.2 percent respectively, while French sales increased only 0.8 percent.

In Germany, sales were solid even though overall discounts were 12 percent of list prices, 0.5 percent less than in June last year and down 0.3 percent from May, analysts at Evercore said.

Last month's sales expansion was driven by mass market manufacturers. Renault and Fiat Chrysler Automobiles' (FCA) Fiat brand, saw sales jump 21.2 percent and 13.9 percent respectively, ACEA figures show.

Mazda <7261.T>, Kia <000270.KS> and Hyundai <005380.KS> also posted double-digit percentage sales gains.

By contrast Volkswagen, Europe's largest carmaker, saw sales of its own brand cars decline 0.6 percent in EU and EFTA markets, ACEA data showed. Volkswagen Group, which also owns the Audi, Skoda, Seat and Porsche brands, saw sales rise 1.5 percent.

VW is grappling with the biggest business crisis in its 78-year history after admitting in September it cheated U.S. diesel emissions tests.

Germany's luxury carmakers also boosted sales, with the core Mercedes <DAIGn.DE> and BMW <BMWG.DE> brand sales jumping 16.1 percent and 14.6 percent respectively, while Volkswagen-owned luxury brand Audi <NSUG.DE> saw flat sales.

Car registrations in Britain, Europe's second-biggest car market, fell 0.8 percent in June, weighed by a 25 percent retreat in cars registered by businesses and a slowdown in private demand.

Passenger car registrations in the European Union rose for the 34th consecutive month, with June registrations reaching 1,459,508 new cars.

The European auto industry association ACEA lifted its 2016 EU sales forecast last month to 14 million passenger cars or 5 percent growth year-on-year, from an earlier forecast of 2 percent sales growth, predicted in January.

(Reporting by Edward Taylor; editing by Christoph Steitz and Susan Fenton)