THERESA May has hit out at Boeing over the escalating US trade dispute with Belfast-linked rival Bombardier, saying its behaviour was "no way to operate".
The prime minister said she was "bitterly disappointed" by the aerospace giant's behaviour and warned its actions "undermined" the long-term partnership it had built up with the UK.
Boeing complained to 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."
DUP leader Arlene Foster said: "Spoke with PM about next steps and the need to work together on this issue. Pleased she gave commitment to work with me to secure Belfast jobs."
Sinn Féin's northern leader Michelle O’Neill said the department's ruling "is a blow to the many families and workers who face a very uncertain future and difficult times ahead".
She added: "A resolution is still possible and I will continue to engage with union leaders, and the Irish and British governments to prioritise the livelihoods of the workers affected by this dispute and secure jobs in Belfast."
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.
British Defence Secretary Sir Michael Fallon warned Boeing that it faces losing out on future contracts.
"This is not the behaviour we expect of Boeing and could indeed jeopardise our future relationship with Boeing," he said on a visit to Belfast.
Boeing said it understood the concerns about jobs in Belfast but claimed Bombardier was not playing by accepted international trading rules.
"Boeing welcomes competition and Bombardier can sell its aircraft anywhere in the world. But sales must be made according to globally-accepted trade rules," it said.
Bombardier, which described the proposed tariff as "absurd", rejected its rival's claims.
Its director of communications in Belfast, Haley Dunne, told the Press Association that the move "is an attack on innovation and competitiveness".
"This is questioning the way that the Canadian government and the UK government have chosen to fund and develop their aerospace and innovations sectors," she said.
The grounds for Boeing's complaint focused on a £135 million investment by Northern Ireland's power-sharing 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.
