The US has complained that Bombardier has dumped its C Series jets at ‘absurdly low’ prices.

The US has escalated its trade dispute with Britain and Canada by announcing plans to slap a further 80% duty on the export of planes built by Bombardier.

The move follows complaints by Boeing that Canadian-owned Bombardier, which employs more than 4,000 people in Belfast, had dumped its C Series jets at “absurdly low” prices.

Bombardier is facing a planned 220% tariff as part of a separate investigation, the US Department of Commerce confirmed.

A second levy of 80% is also being applied to Bombardier’s sales to the US after a preliminary finding that the jets were sold below cost price to Delta Air Lines in 2016.

Boeing claimed that 75 aircraft were sold at nearly £10.6m below cost price. Bombardier dismissed the claim as “absurd”.

The company is due to begin delivering a blockbuster order for up to 125 new jets to Atlanta-based Delta next year.

The 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.”

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 Department of Commerce said Bombardier had failed to provide the information it had requested.

The US government’s preliminary decision affects imports of 100 to 150 aircraft from Canada.

The department said it would instruct US Customs and Border Protection to collect cash deposits of duties.

The proposed duties would not take effect unless affirmed by the US International Trade Commission (ITC) early next year. To win its case before the ITC, Boeing must prove it was harmed by Bombardier’s sales, despite not using one of its own jets to compete for the Delta order.

Bombardier said it was confident that the ITC would find Boeing had not been harmed, calling the Department of Commerce decision a case of “egregious overreach”. Delta said the decision was preliminary and it was confident the ITC “will conclude that no US manufacturer is at risk” from Bombardier’s plane. Boeing has said the dispute was not an attack on Canada or Britain.

The UK prime minister, Theresa May, lobbied the US president, Donald Trump, last month over the dispute sparked by Boeing’s complaints about “unfair” state subsidies from the UK and Canada. She also discussed the issue with her Canadian counterpart, Justin Trudeau.

The UK business secretary, Greg Clark, travelled to Chicago for a meeting with Boeing’s chairman, president and chief executive, Dennis Muilenburg, to try to solve the dispute.

The decision is expected to heighten trade tensions which flared last week after the US announced a preliminary duty of nearly 220% for subsidies Bombardier received, which was well above the 80% duty Boeing sought in its complaint. Unions have warned that the move could put jobs in jeopardy.

In retaliation, Canada and Britain threatened to avoid buying Boeing military equipment, saying the duties on the C Series would reduce US sales. The combined 300% duty would effectively keep the plane out of the market.

Bombardier shares dipped 2 cents to C$2.18 immediately after the announcement but have since recovered.