Mrs May, who has directly lobbied US President Donald Trump over the dispute, expressed regret at the decision.
"The simple truth is that Bombardier created a superior aircraft that is more efficient, more comfortable, and quieter", said the company. I think this is no way to operate in such a long-term partnership.
"The magnitude of the proposed duty is absurd and divorced from the reality about the financing of multibillion-dollar aircraft programs", said Bombardier in a statement following the Commerce Department's announcement. The exact amount of any new tariffs would not likely be finalized until early next year. This will mean lower profits for US airlines and higher prices for travellers. Bombardier's Northern Ireland factory, which employs 4,200 people, produces 10% of the region's total manufacturing exports.
Canada's giant multinational aerospace and transportation company, Bombardier Inc., received a giant blow from the U.S. Department of Commerce on Tuesday. But British and Canadian leaders have publicly urged the Trump Administration not to impose tariffs.
"Boeing stand to gain a lot of British defence spending".
Canada is in the midst of negotiations to buy 18 Boeing-made F/A-18 Super Hornet fighter jets for an estimated $5.15 billion.
Speaking to Sky News she said: "Unfortunately, it's not a surprise".
Some have questioned why Boeing would wade into global geopolitics in the first place.
Analyst Richard Aboulafia of the Teal Group said Bombardier has no choice but to stay the course and hope things get better.
"The only way Boeing could help the CSeries in some markets is by doing this", he said.
However, London's options in fighting Bombardier's corner may be limited because of the importance of Boeing to its defence industry.
The premier of Quebec province urged Canada on Wednesday to take a tough stance against Boeing after the USA slapped a 219 percent tariff on airplanes sold to Delta Air Lines by Montreal-based Bombardier, APA reports quoting Anadolu Agency.
The company has doubled its direct employment in the United Kingdom since 2011 and tripled its spending with more than 250 companies in the United Kingdom supply chain over the same period, to £2.1bn in 2016.
The US decision could potentially create long-lasting damage for Boeing in the Canadian and United Kingdom markets, both large buyers of the kinds of defense and aerospace systems manufactured by the company. "This dispute has nothing to do with limiting innovation or competition, which we welcome". The aerospace firm said it only wants to ensure all businesses in the sector abide by the rules.
"Although couched in terms of fair-trading practices by U.S. Commerce Secretary Wilbur Ross, these decisions could have serious negative consequences for both Bombardier and the North American economy".