Nissan Motor Co. gave the starkest warning yet on the future of the Japanese group's car factories in Western Europe, with a plant in the U.K. threatened by Brexit and another in Spain suffering from a slump in demand.

The Sunderland site in England, which makes models that account for the bulk of European sales, remains under a cloud of uncertainty, Gianluca de Ficchy, chairman of Nissan Europe, said Monday in a press conference near Paris.

Should Britain fail to reach a free-trade agreement with the European Union, a resulting 10 percent tariff on cars and parts could not only spell the demise of the plant, which sends about three-quarters of its output to the continent, but also of Nissan's entire European strategy, the executive said.