By Andreas Cremer

BERLIN (Reuters) - Ford <F.N> warned on Thursday that potential future investment in the United Kingdom could be at risk if a vote to leave the European Union leads to a significant deterioration in the country's business environment.

Ford is turning the corner in Europe after years of losses, recording its highest quarterly pretax profit since 2008 in the first three months of 2016 as it reduces personnel costs and sells more higher-margin cars.

In an interview with Reuters, Ford Europe Chief Executive Jim Farley said the U.S. automaker's "primary interest" was to maintain stability to increase its business further in the United Kingdom, its largest market in Europe.

"If the UK left the EU, it could create economic instability and uncertainty, the full consequences of which are unknown generally and, specifically, to our business," Farley said.

He declined to say whether Ford had a contingency plan in place if the United Kingdom votes to leave on June 23 but indicated that investment could be at risk if it withdrew.

"Should a vote to leave lead to a significant deterioration in the UK's business conditions and trade environment then that could impact business decision-making, including potential future investment," the CEO said.

The U.S. automaker has already sent a letter to its 14,000 UK employees saying it would be better if Britain, which is Europe's second biggest car market, stays in the EU.

Campaigners seeking to keep Britain in the EU have also said the country's role as a global financial hub could be under threat if it gives up its membership, resulting in job losses.

Farley, an American who has led Ford's operations in Europe since January 2015, wants the EU to improve regulations to make industries more competitive and to rework a free trade deal with South Korea that he says discriminates against mass-market carmakers such as Ford.

"We believe the UK staying as a member of a reformed EU is the best way of maintaining stability and avoiding uncertainty in the trading environment," he said.

The U.S. automaker's operations in Britain include three plants making engines and transmissions, a technical center in Dunton with 3,000 employees, the Ford Credit Europe (FCE) banking arm and a dealer network with 600 service points.

The FCE, which offers retail, leasing and other financial services to Ford dealers and customers in 15 European countries, depends on its ability to operate across the region using an EU passporting arrangement, Farley said.

(Editing by David Clarke)