Bombardier is facing a 300% duty on its exports of planes to the US after the US government proposed adding a further 80% tariff in an international trade dispute.
One of Northern Ireland's largest employers is already facing a planned 220% tariff on its aircraft as part of a separate investigation, the US Department of Commerce confirmed.
The Canadian-owned Bombardier employs more than 4,000 people at its Belfast factories and is due to begin delivering an order for up to 125 new jets to Atlanta-based Delta Airlines next year.
The dispute was sparked by complaints from rival Boeing that Bombardier received unfair state subsidies from the UK and Canada, allowing 'dumping', or the export of a product at a lower price.
Unions have warned thousands of jobs could be in jeopardy.
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."
Theresa May has lobbied US president Donald Trump over the levy, but the additional tariff was added after Bombardier had failed to provide information requested, the Commerce Department said.
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."
The US government preliminary decision affects imports of 100-150 aircraft from Canada.