GENEVA—Global auto makers aren’t yet prepared to confront the unthinkable—a British exit from the European Union—but industry executives gathering at the Geneva Motor Show warned Tuesday that such a decision would have a significant impact on sales and jobs.

The U.K., famous for brands such as Aston Martin, the iconic sports car in James Bond movies, as well as Bentley, Rolls-Royce and Jaguar luxury vehicles, is the EU’s second-largest automobile market, posting sales of 2.4 million vehicles last year. The U.K. trails Germany but is ahead of Italy, France and Spain, the five countries that account for the lion’s share of EU car sales.

Jim Farley, president of Ford Motor Co. ’s Europe, Middle East and Africa business, said a U.K. vote to leave the 28-member bloc would be “unprecedented.” Ford was one of 100 European businesses to sign a declaration last week urging the U.K. to stay in the EU to protect jobs that depend on access to European markets. He said British opinion may be shifting in favor of staying, but he declined to make a prediction on how U.K. citizens would vote in the June 23 referendum.

“It’s too early to say,” he told reporters in Geneva on Tuesday. “For employers, access to a single market is absolutely critical.”

Mr. Farley’s view was echoed by other auto executives who cited the potential effect on the British currency, jobs, and the economy if the U.K. lost free access to European markets and manufacturing costs surged because of more expensive imports.

Dieter Zetsche, chief executive officer of Daimler AG, said it would be “very, very regrettable” if the U.K. were to leave the EU.

Harald Krüger, CEO of BMW AG , also said he would personally regret a U.K. exit but said any prediction about how it would impact the auto industry and the broader European economy is “pure speculation” at this point.

“It’s hard to tell what it could mean,” he said. “BMW has MINI, Rolls-Royce and an engine factory in the U.K,” adding that if it left, the U.K. would need to negotiate a trade agreement to dull the effect of its secession.

In June, British voters face a referendum asking whether the U.K. should stay in the EU or leave the bloc. The issue has divided the British Conservative party, with Prime Minister David Cameron calling for the U.K. to remain and other top party officials coming down on the other side. Boris Johnson, a fellow conservative believed to be a possible contender for the party leadership in the future, said last week he supports exiting the EU.

Some observers say an exit wouldn’t be all bad. There could be an upside for local exporters, for example, that could gain a competitive advantage from a weaker pound against other currencies.

Even some car executives played down the potential implications on the auto industry.

Sergio Marchionne, chief executive of Fiat Chrysler Automobiles NV, said while an eventual British exit would have ramifications across Europe and be “unhelpful for the European project,” the effects on the Italian-American car maker and its competitors wouldn’t be particularly pronounced.

“We have already had to adjust to the fact that while the U.K. is a major European car market, it doesn’t use the euro,” said Mr. Marchionne. “We would adjust, but I’m not sure it would change too much about the way we confront the British market. It has wider implications beyond an auto producer.”

But other auto makers warned that the weaker British pound, which is already under pressure against the euro, poses dangers for the country’s economy.

“Prices will increase to compensate for the weak currency,” said Carlos Tavares, CEO of French auto maker PSA Peugeot Citroën.

In addition to potential economic fallout, auto makers warned that Britain’s exit from the deprive the EU of a strong, economically liberal voice that has often prevented the EU from swinging too far to the left.

“I would find it very regrettable,” said BMW’s Mr. Krüger, adding that the U.K. is a “driver for market liberalization and other reforms.”