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 last night.

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 major order for up to 125 new jets to Atlanta-based Delta Airlines next year.

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."

British Prime Minister Theresa May had lobbied US 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.

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 anti-dumping 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.

The wings for the new aircraft, which are due to be delivered to the US next year, are made at Bombardier's plant in the DUP stronghold of East Belfast.

The Northern Ireland party's ten MPs are propping up Theresa May’s minority administration in the House of Commons in the UK and are expected to play a crucial role during upcoming Brexit business in parliament.

The alleged unfair subsidy arose after Northern Ireland's power-sharing administration and the UK government pledged to invest almost £135m in the establishment of the C-Series manufacturing site in Belfast.

The programme also received $1bn from the Canadian provincial government in Quebec in 2015 when its fortunes appeared to be ailing.

Boeing's complaint said it was seeking a "level playing field" for global competitors, but Bombardier accused its rival of hypocrisy.

Boeing says it is the world's largest manufacturer of military aircraft.

Whether its payments from governments constitute a purely commercial matter is contested.

The C-Series operation's immediate future was thought to have been secured after Bombardier signed a $5.6bn deal in 2016 to provide the aircraft.

The manufacturer has been a major employer in Northern Ireland for 30 years.

Trade unionists expect a final ruling on the pricing policy to be made in February.