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She said that Boeing’s long-term partnership with the government was being “undermined by this behaviour”.

“We are very clear about the importance of Bombardier and the importance of those jobs in Northern Ireland and we will be doing everything we can to ensure that we can see those jobs being guaranteed in future,” she added. Echoing Mrs May’s comments, Sir Michael Fallon, the Defence Secretary, said: “This is not the kind of behaviour we expect from a long-term partner.

“We have contracts in place with Boeing for new maritime patrol aircraft and for Apache attack helicopters and they will also be bidding for other defence work and this kind of behaviour clearly could jeopardise our future relationship with Boeing.”

Photo by PAUL FAITH/AFP/Getty Images

May has ordered ministers to “engage intensively” with Boeing and the U.S. administration to try to resolve the matter. The dispute centres on a Boeing complaint that the jets were being sold in the U.S. at low prices after unfair state subsidies from the U.K. and Canada had helped Canadian-owned Bombardier win an order for up to 125 aircraft with the American airline Delta.

The tariffs will only take effect if the U.S. International Trade Commission rules in Boeing’s favour in a final decision expected next year. But the interim tariffs were recommended despite May asking Trump to intervene.

Arlene Foster, the DUP leader, insisted the government must challenge the “unjustifiable” ruling. Foster, upon whose party the government relies to secure key Commons votes, has repeatedly pressed May on the issue. She told Sky News: “Unfortunately, it’s not a surprise. What we must do now is to continue to work with our own Government, with the American government, with the Canadian government, in trying to get Boeing to see sense.