Aerospace giant Airbus Group has written to its 15,000 UK employees warning of the risks of a Brexit vote.
Bosses sent the letter on Monday outlining fears that leaving the European Union could increase competition in the UK market.
The aircraft manufacturer employs 136,000 people globally and 15,000 people in the UK.
While it said it remains "committed" to its UK operations whatever the outcome of the referendum, it said its success is based on a "highly competitive, integrated European business model".
The letter, signed by six top Airbus Group bosses, including the president of Airbus Group UK Paul Kahn, said: "We simply don't know what 'out' looks like."
It said the bosses "firmly believe that it makes good economic sense to stay inside the EU".
It continued: "Should the British electorate have a different view then clearly we wouldn't cease our activities in the UK, which are highly important and very prominent.
"However, our business model is entirely based on our ability to move products, people and ideas around Europe without any restriction and we do not believe leaving will increase the competitiveness of our British based operations.
"We all need to keep in the back of our minds that future investments depend very much on the economic environment in which the company operates."