The wings of Bombardier's CSeries aircrafts are made in a Belfast factory employing more than 4,000 people Eric Piermont/AFP/Getty Images

The US government has ruled that Canadian aircraft manufacturer Bombardier had benefitted from government subsidies which allowed it to sell its C-Series jets below cost in the US.

In September, the US government imposed an import tax of 80% on Bombardier's C Series jets, on top of a 220% tariff already set, after aerospace giant Boeing complained the Canadian company was selling its planes at "absurdly low" prices.

Boeing claimed Delta Air Lines paid $20m per plane when it purchased 75 C-Series jets, well below an estimated cost of $33m and what Bombardier charges in Canada.

The wings of C-Series jets are produced in Belfast and, according to Boeing, any form of aid from the Canadian and British governments amounts to an illegal subsidy.

On Wednesday night (20 December), the US Commerce Department ruled in favour of the US aerospace giant and set trade duties of approximately 292%, just marginally lower than it had done in a preliminary finding.

Bombardier said it was "deeply disappointed" by the decision, adding the Commerce Department had not taken into accounts its plans to build a facility in the US, a part of its partnership with Airbus.

"This facility will provide US airlines with a US-built plane thereby eliminating any possibility of harm due to imports," said the company's spokesman Mike Nadolski.

"Unfortunately, the Commerce Department decision is divorced from this reality and ignores long-standing business practices in the aerospace industry, including launch pricing and the financing of multibillion dollar aircraft programs."

The inquiry is now in the hands of the US International Trade Commission, which will have to establish whether the subsidies effectively hindered Boeing. The commission is expected to make a decision in February next year, which would trigger the duties.

Bombardier is one of the largest private sector employers in Northern Ireland, with operations spanning across four different plants, and the ruling could have a significant impact on its operations in the country.

The Canadian company employs 4,000 staff in total in Northern Ireland, a quarter of whom work on the C-Series production chain.

In October, Bombardier workers called on Theresa May and her government to do more to safeguard the jobs, after the government was accused of failing to go beyond "fluffy words" to ensure operations at the Belfast plant will not be affected.

The Unite union described the Commerce Department decision "nakedly political", warning it could "crush jobs, not only in Northern Ireland but in the US too".

"More than 50% of C-Series components are sourced from the US, where the supply chain sustains 22,000 US jobs," said Unite assistant general secretary Steve Turner.

"The economic impact of these tariffs would be felt in communities on both sides of the Atlantic. Boeing is using its meritless complaint as cover to close the US market, which is one of the biggest in the world, to new entrants such as Bombardier's C-Series aircraft."