Airbus, the world's second-largest planemaker after Boeing, makes wings for all its passenger jets at a 2 billion-pound factory in Broughton, in north Wales. It get 6 billion pounds of revenues a year in the U.K. as a whole.
The warning came as Britain's largest business lobby group urged bosses to defend membership of the European Union by telling voters it is the best guarantee of prosperity.
Vodafone's chief executive has said the UK ought to remain a member. On the other hand, JCB Chief Executive Graeme MacDonald says it would not make much difference in trade with the rest of Europe.
Airbus is one of the first industrial companies to speak out in favor of EU membership since this month's UK election, which saw Prime Minister David Cameron's Conservatives win a majority.
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Cameron has pledged to renegotiate Britain's ties with Europe and then give voters a referendum on whether to leave the EU by the end of 2017.
Airbus's Kahn said the long-term economic risks of a potential British exit were "huge".
"In Wales alone, some 150-200,000 jobs could be directly impacted if Britain were to leave," he said, according to a text of his remarks issued by Airbus.
"If after an exit from the European Union, economic conditions in Britain were less favorable for business than in other parts of Europe, or beyond; would Airbus reconsider future investment in the United Kingdom? Yes, absolutely".
He told the Welsh audience other European nations wanted to "steal our lunch" in aerospace. He did not name specific countries, but industry experts say Germany and Spain - two other partners in Airbus, along with France - have long coveted wings manufacturing because it can generate highly skilled jobs.
Kahn said the long lead times needed to build up the skills to support advanced manufacturing meant that jobs would be slow to leave. But they could be "even slower to return" should companies choose to send their future investment elsewhere.