One of Northern Ireland’s largest employers is facing a proposed 300% duty on its exports of planes to the US amid an international trade dispute, the US government said on Friday.

A second preliminary levy of 80% has been loaded on the sales of aerospace manufacturer Bombardier.

The Canadian-owned multinational is already facing a planned 220% tariff on its aircraft as part of a separate investigation, the US Department of Commerce confirmed.

Bombardier employs more than 4,000 people at its Belfast factories and is due to begin delivering a blockbuster order for up to 125 new jets to Atlanta-based Delta Airlines next year.

Wilbur RossUS commerce secretary Wilbur Ross (AP Photo/Vincent Yu)

US commerce secretary Wilbur Ross said: “The United States is committed to free, fair and reciprocal trade with Canada, but this is not our idea of a properly functioning trading relationship.

“We will continue to verify the accuracy of this decision, while doing everything in our power to stand up for American companies and their workers.”

Prime Minister Theresa May had lobbied President Donald Trump over the dispute sparked by complaints from rival Boeing that Bombardier received unfair state subsidies from the UK and Canada, allowing the sale of airliners at below cost prices in the US.

Theresa May and Donald TrumpTheresa May had lobbied US President Donald Trump over the dispute (Stefan Rousseau/PA)

Unions have warned thousands of jobs could be in jeopardy.

The US government said its intervention was prompted by concern to prevent “injurious dumping” of imports into the country, “establishing an opportunity to compete on a level playing field”.

The Commerce Department said Bombardier had failed to provide information requested.

It added: “The antidumping duty law provides US businesses and workers with a transparent, quasi-judicial, and internationally accepted mechanism to seek relief from the market-distorting effects caused by injurious dumping of imports into the United States.”

Dumping means the export of a product at a lower price.

The US government preliminary decision affects imports of 100-150 aircraft from Canada.

The department said it will instruct US Customs and Border Protection (CBP) to collect cash deposits of duties.