The Prime Minister has echoed comments made by defence secretary Michael Fallon over the Bombardier tariff row with Boeing, saying today that the American firm was undermining its relationship with Britain.
Speaking at a Bank of England (BoE) event today, May said: "What I would say in relation to Boeing is that of course we have a long-term partnership with Boeing in various aspects of government and this is not the sort of behaviour we expect from a long-term partner and undermines that partnership."
On Tuesday, the US Department of Commerce imposed a 219.63 per cent duty on Bombardier's CSeries jets in a preliminary finding, following a complaint by Boeing accusing the Canadian firm of receiving unfair state subsidies.
Boeing said the dispute had "nothing to do with limiting innovation or competition", and was instead to do with "maintaining a level playing field and ensuring that aerospace companies abide by trade agreements".
Bombardier has branded the ruling "absurd", and concerns have been raised in the UK that thousands of jobs will be put at risk by the ruling. The wings on the CSeries are made in Belfast, and trade unions have warned that tariffs could make Bombardier question whether to stay in Northern Ireland, where it employs over 4,000 people.
And the government has hinted that Boeing's actions could put future defence contracts at risk.
Speaking to reporters in Belfast yesterday, Fallon said: "This is not the behaviour we expect from Boeing and it could indeed jeopardise our future relationship with them."
"Boeing has significant defence contracts with us and still expects to win further contracts. Boeing wants, and we want, a long-term partnership, but that has to be two-way."
The government has said it is "doing everything we can" to ensure the Northern Ireland jobs are guaranteed in the future.
Boeing said yesterday: "We have heard and understand the Prime Minister’s concerns about Bombardier workers in Northern Ireland.
"The company has doubled its direct employment in the UK since 2011 and tripled its spending with more than 250 companies in the UK supply chain over the same period, to £2.1bn in 2016.
"We are pleased to work with our partners, including the British government, and provide such a vote of confidence in the UK."