The British government said Wednesday it was "bitterly disappointed" by a US decision to impose stiff tariffs on Canadian aerospace firm Bombardier, which employs thousands of people in Northern Ireland.

In a preliminary ruling, the US Commerce Department said it was imposing anti-dumping duties of 220 percent on the firm's CSeries jets following a complaint by American manufacturer Boeing.

"Bitterly disappointed by initial Bombardier ruling. The government will continue to work with the company to protect vital jobs for Northern Ireland," Prime Minister Theresa May's government said on Twitter.

Bombardier employs 4,200 in aeronautics, most in a Belfast factory that builds the CSeries wings and fuselage, and thousands more in the supply chain.

Ross Murdoch, of the GMB union, accused the prime minister of being "asleep at the wheel when she could and should have been fighting to protect these workers. It's high time she woke up".

In a statement, the government's business department said the ruling that Bombardier had unfairly benefited from state subsidies was "only the first step in the process".

"We will continue to strongly defend UK interests in support of Bombardier at the very highest level because an adverse outcome risks jobs and livelihoods among the 4,200 skilled workers in Belfast," it said.

"Boeing's position in this case is unjustified and frankly not what we would expect of a long-term partner to the UK -- as well as damaging the wider global aerospace industry.

"We will continue to work closely with the Canadian government to encourage all parties to reach a credible resolution as quickly as possible."

Canada has also strongly protested against the ruling, which is set for a final determination on December 12.

Arlene Foster, the leader of the largest party in Northern Ireland, the Democratic Unionists, also said the decision was "very disappointing".

"The CSeries is a hugely innovative aircraft that is vital to Bombardier's operations in Belfast. It is this innovation that sets the C-Series apart and it is not in direct competition with Boeing," she said.