LONDON, March 3 (Reuters) - Germany’s BMW has written to the British employees who make its luxury Rolls-Royce car to warn about the risks the firm would face if Britain votes to leave the European Union at the upcoming referendum, the BBC said on Thursday.

In a letter obtained by the corporation, the German carmaker set out what it sees as the benefits from Britain’s membership of the 28-member bloc and the problems it would face if Britain votes to leave on June 23.

“The decision on whether to stay in the EU or not is for British voters to decide on in June,” the letter said. “However, as a wholly-owned BMW Group company, it is important for all Rolls-Royce Motor Cars employees to understand the view of our parent company.”

The firm said it was not clear that an independent Britain would be granted a free trade deal with the EU, meaning tariff barriers could lead to higher costs and higher prices.

It also said that the free movement of people between Britain and the EU had led to the rapid transfer of knowledge between Rolls and BMW, helping to improve the skill level of the British workforce.

Prime Minister David Cameron, who is campaigning to keep Britain in the bloc, has called on big business to explain the benefits of EU membership. In February, the bosses at more than a third of Britain’s biggest companies joined forces to warn that leaving the EU would put the economy at risk.

However many companies held back from signing the letter, saying they wished to remain neutral in such an important debate.

Several big companies were criticised in the run up to the 2014 Scottish independence referendum when they urged Scots to stay in the United Kingdom, with some voters saying they would boycott those firms considered to be interfering in the country’s political debate.

BMW was not immediately available for comment. (Reporting by Kate Holton, editing by Andy Bruce)