Union leaders are warning that hundreds of jobs are under threat in Belfast because of a decision by the United States to impose duties on imports of Bombardier C series commercial jets.

Following a petition by the US aeroplane manufacturer, Boeing, the US Commerce Department began an investigation into Bombardier.

It found that the Canadian company received government subsidies and sold C-Series jets below cost in the US.

A final decision will be made in February that might result in duties of almost 300% being imposed on Bombardier products.

The company employs about 1,000 people in Belfast linked to the C-Series.

Unite regional officer for Bombardier in Northern Ireland, Susan Fitzgerald, said: "This decision poses a devastating risk for the Northern Ireland economy.

"Tariffs on the scale proposed by the US Commerce Department in the world's largest airline market, threaten to undermine the long-term economics of Bombardier's presence in Northern Ireland.

"Not only is it a threat to the economy, it poses a significant threat to stability in Northern Ireland. Unite is very conscious that workplaces are the largest integrated environments in our society."

Mike Nadolski, Bombardier's Vice President of Communications and Public Affairs, said evidence presented by Boeing earlier this week at the US International Trade Commission demonstrated an "unfounded assault" on airlines, the flying public, and the US aerospace industry.

He added: "That has been true since the start of the investigation, and recent developments make it even clearer, particularly the Bombardier and Airbus partnership, which will include the construction of a new US manufacturing facility in Alabama.

"This facility will provide US airlines with a US-built plane thereby eliminating any possibility of harm due to imports.

"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.

"Moreover, we are deeply disappointed that the Commerce Department did not take this opportunity to rectify its past errors.

"We remain confident that at the end of the process, the United States International Trade Commission will reach the right conclusion, which is that the C Series benefits the US aerospace industry, US airlines, and the US flying public.

"The fact is that the C Series simply does not threaten Boeing."