One of the world's largest aerospace firms has written to its 15,000 UK employees warning about the risks of voting to leave the European Union.
In a letter to staff, Airbus Group said it makes "good economic sense" for the UK to stay in the EU due to its ability to trade freely and recruit workers.
While it would not relocate elsewhere in the event of EU exit, the company said it might reduce its investment.
The campaign to quit the EU said it was "another example of Project Fear".
Conservative MP Peter Bone of Grassroots Out, one of groups vying to be the official Leave campaign, said Airbus was ignoring the economic risks of staying in the EU, which he described as a "club that stops us from making our own trade deals with the rest of the world".
Airbus employs 15,000 workers at 25 sites across the UK, generating £6bn a year making large aeroplanes and helicopters for civilian airlines and the RAF as well as satellites and cyber defence equipment.
The company says it is indirectly responsible for a further 100,000 jobs among its British suppliers.
It is no surprise that Airbus - which has large operations in France, Germany and Spain as well as Britain - supports the UK remaining in the EU. The company signed a pro-Remain letter to The Times with many other businesses in February.
But what is significant is its decision to come to a clear corporate view and express that view to its staff. Many businesses choose to stay out of the debate or simply allow their bosses to speak out in a personal capacity.
Paul Kahn, the president of Airbus Group UK, told me: "We are not telling our staff how to vote, we are trying to explain to them why the company has the position that we do have."
Asked whether the company would move from the UK if there was a vote to leave the EU, he said: "We are not going anywhere" but warned investment could be diverted to China and the United States.
In the letter, the company emphasises what has helped to make the company a success because of its ability to move people and products around Europe.
"Airbus Group's success in the UK is predicated on a highly competitive, integrated European business model," it says.
"Our position... is we firmly believe that it makes good economic sense to stay inside the EU which has helped make the company the global success story it is today. Apart from anything else, we simply don't know what 'out' looks like."
It continues: "It is also reasonable for us as a responsible UK leadership team to explain to you why we think it is important for us to stay. 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.
"In fact we believe that the UK remaining active in reforming the EU can improve our position, and as a commercial business that operates in a globally competitive environment we need to maintain every competitive advantage in the UK we have."
But Mr Bone said many leading business figures did not support Airbus's stance, adding: "Many firms who struggle with relentless interference from the EU and rules that are stacked in the favour of a select few multi-nationals say we will be better off out."
Britain Stronger in Europe, which is campaigning for the UK to stay in the EU, said Airbus had an "absolute right" to speak out, describing the company as "a shining example of how being in the EU creates jobs and opportunities in Britain".