Theresa May has hit out at Boeing over the escalating US trade dispute with Belfast-linked rival Bombardier, warning its behaviour was "no way to operate".
The British Prime Minister said she was "bitterly disappointed" by the aerospace giant's behaviour and its actions "undermined" the long-term partnership it had built up with the UK.
Boeing complained to the US authorities about state subsides paid to Canadian manufacturer Bombardier by the UK and Canada.
Its petition resulted in a ruling by the US Department of Commerce that could potentially have a devastating impact on Bombardier's 4,200 workforce in Northern Ireland and thousands more in the 800-plus UK and Irish companies involved in its supply chain.
The department has proposed a 220% tariff on the imported sale of Bombardier's new C-Series jets into the US - an aircraft whose wings are made in Belfast.
Mrs May said: "I'm bitterly disappointed by this news."
"I will be doing everything I can, as the government has been, and working with both Arlene Foster and Michelle O'Neill - I spoke to them both today about this decision - to ensure that we can try to make sure the future of Bombardier in Northern Ireland is guaranteed and protected."
"We have had a long-term partnership with Boeing, worked with Boeing over the years, and I think this is no way to operate in terms of such a long-term partnership.
"I would say that that long-term partnership is being undermined by this behaviour by Boeing."
The US International Trade Commission will decide in February whether to uphold or reject the proposed tariff.
Boeing currently has defence contracts with the UK worth around £8 billion.
Britain's Defence Secretary Michael Fallon earlier warned Boeing it faces losing out on future UK defence contracts.
"This is not the behaviour we expect of Boeing and could indeed jeopardise our future relationship with Boeing," Mr Fallon said on a visit to Belfast.
"Boeing stand to gain a lot of British defence spending. 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."
The UK's Ministry of Defence said while future deals could be jeopardised, it stressed existing contracts with Boeing would be honoured.
The DUP, which the Tories rely on to swing key Commons votes for the British Government, has repeatedly pressed the Prime Minister over the Boeing/Bombardier issue.
Mrs May has directly lobbied US President Donald Trump over the dispute and has spoken to Canadian Prime Minister Justin Trudeau.
Boeing said it understood the Government's concerns about jobs in Belfast but claimed Bombardier was not playing by accepted international trading rules.
It also stressed that it employed more than 18,000 people either directly or through its supply chain in the UK and had chosen to build its first European factory in Sheffield.
"Boeing is committed to the UK and values the partnership, which stretches back almost 80 years," said a company statement.
It added: "Boeing welcomes competition and Bombardier can sell its aircraft anywhere in the world. But sales must be made according to globally-accepted trade rules."
Bombardier, which described the proposed tariff as "absurd", rejected its rival's claims.
Bombardier's director of communications in Belfast Haley Dunne said that Boeing was trying to undermine the right of governments to invest in industry.
"This is an attack on innovation and competitiveness," she said.
"This is questioning the way that the Canadian government and the UK government have chosen to fund and develop their aerospace and innovations sectors."
The grounds for Boeing's complaint focused on a £135 million investment by Northern Ireland's powersharing administration and the UK Government to establish the C-Series manufacturing site in Belfast.
It also cited the one billion US dollars Bombardier received from the Canadian provincial government in Quebec in 2015 when its fortunes appeared to be ailing.
The future of the C-Series jets appeared secure after Bombardier signed a US$5.6bn deal in 2016 to provide the aircraft.
The manufacturer, which has been a major employer in Northern Ireland for 30 years, is due to begin delivering a blockbuster order for up to 125 new jets to Atlanta-based Delta Airlines in 2018.
Announcing the regulator's preliminary finding on Tuesday, US Secretary of Commerce Wilbur Ross said the subsidisation of goods by foreign governments was something that Mr Trump's administration "takes very seriously".