Engineering giant Rolls-Royce has written to its employees telling them it wishes to remain the European Union.
Brexit would "limit any company's ability to plan and budget for the future," the firm said.
Rolls-Royce currently employs 37,000 people -3/4 of its total workforce - in the European Union. It provides about 200,000 jobs across the trading bloc.
Rolls-Royce chief executive Warren East said Brexit their big American rivals a competitive advantage.
Warren East said:
"It's all about uncertainty and our position in Europe.
"We have a very interconnected operation around Europe... We're making investment decisions all the time about where to place various parts of our operation... and uncertainty created by Brexit puts a lot of those decisions on hold, and that pause is something that our US competitors don't have to cope with."