Prime Minister Theresa May has phoned US President Donald Trump over a threat to jobs at Bombardier in Belfast from a trade dispute.
In 2016, Canadian firm Bombardier won an order to supply up to 125 C-Series passenger jets to US airline Delta.
The wings for the C-Series are made at Bombardier’s Belfast plant.
However, rival aircraft firm Boeing has complained to the US authorities that the deal was unfairly subsidised by the Canadian state.
Boeing has also complained about a UK government loan made to the Bombardier plant in Belfast.
The US Department of Commerce is due to make a ruling later this month.
It could hit Bombardier with punitive tariffs.
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Mrs May and her Canadian counterpart, Justin Trudeau, would discuss the dispute at a meeting in Ottawa on 18 September, Reuters reported.
Tariffs could make it very difficult for Bombardier to find new C-Series customers in the US.
The C-Series project supports hundreds of jobs in Belfast.
Phone call to Trump
The government was “working tirelessly to safeguard Bombardier’s operations and its highly skilled workers in Belfast”, said a spokesperson.
“Ministers across government have engaged swiftly and extensively with Boeing, Bombardier, the US and Canadian governments,” added the spokesperon.
“Our priority is to encourage Boeing to drop its case and seek a negotiated settlement with Bombardier.”
Mrs May raised the issue and her concern to protect jobs in Northern Ireland in a call with President Donald Trump last week, Downing Street confirmed.
Business Secretary Greg Clark had also travelled to Chicago to meet Boeing’s chairman, president and chief executive officer, Dennis Muilenburg.
Bombardier managers in Belfast are also understood to have recently briefed trade unions about the importance of the case.
Boeing has alleged that Bombardier engaged in “price dumping” by agreeing to sell 75 of their planes for almost $14m (£10.6m) below their cost price.
The company said it had appealed to the International Trade Commission “to restore a level playing field in the US single-aisle airplane market”.
“Boeing had to take action as subsidised competition has hurt us now and will continue to hurt us for years to come, and we could not stand by given this clear case of illegal dumping,” it said in a statement.
“Global trade only works if everyone plays by the same rules of the road, and that’s a principle that ultimately creates the greatest value for Canada, the United Kingdom, the United States, and our aerospace industry.”
It also pointed out that the Delta deal came after the regional government in Quebec effectively bailed out the C-Series programme with a $1bn investment.
“Equity infusions from government coffers not only rescued the program but have given Bombardier the resources it needs to aggressively target the US market,” it said.
Bombardier has described the allegations as “absurd” and said the government investments “comply with the laws and regulations in the jurisdictions where we do business”.
Speaking in the House of Commons on Tuesday, Liberal Democrat leader Sir Vince Cable called on the UK government to “commit itself to standing very firmly behind Bombardier and its workers, and alongside the Canadian Government in resisting bullying from Boeing and its friend in the United States administration”.
Minister for Climate Change and Industry Claire Perry said: “It is vitally important that we have this dispute settled and we create the environment for many manufacturers in this vital sector to thrive and grow.”
Strangford DUP MP Jim Shannon raised concerns about the future of the C-Series with Research and Innovation Minister, Jo Johnson.
“He’ll be aware of Boeing’s attempts to stop the contract which will add $30m (£23m) to every plane (coming into) C-Series in Belfast,” he said.
Mr Johnson replied: “I can assure him that we are engaging very closely with the companies involved and will be following up on his point.”