(Adds reaction from Boeing)
OTTAWA, Sept 18 (Reuters) - British Prime Minister Theresa May said on Monday she would press U.S. President Donald Trump this week about a trade challenge by Boeing Co that could endanger thousands of aerospace jobs in Northern Ireland.
May and Prime Minister Justin Trudeau are worried about Boeing's move against Bombardier Inc, which is the single largest manufacturing employer in Northern Ireland and Canada's most important aerospace firm.
May raised the issue with Trump in a call earlier this month and told reporters in Ottawa she would do so again this week on the margins of the United Nations.
"I will be impressing on him the significance of Bombardier to the United Kingdom ... I want to see a resolution that protects those jobs in Northern Ireland," she said after talks with Trudeau where both leaders agreed to work together to stress that Boeing should back down.
May's minority Conservative government depends on backing from the small Northern Irish Democratic Unionist Party (DUP) for their majority control of the British parliament.
Boeing accuses Bombardier of dumping its new CSeries passenger jet in the U.S. aircraft market, a charge the Canadian firm denies.
A U.S. trade court is due to give a preliminary ruling on Boeing's complaint on Sept. 25.
"I am very happy to be working with Prime Minister May to explain to the American administration how Boeing's actions are harmful to workers here in Canada," Trudeau told reporters.
Trudeau reiterated that Canada would not talk to Boeing about a proposed purchase of 18 F-18 Super Hornet fighter jets until the firm had dropped its challenge.
"We won't do business with a company that's busy trying to sue us and put our aerospace workers out of business," he said.
Boeing denied it was suing Canada.
"This is a commercial dispute with Bombardier, which has sold its CSeries airplane in the United States at absurdly low prices ... this is a classic case of dumping," it said in a statement.
Trudeau's office did not immediately respond to a question about his use of the word "suing."
At the news conference, May sidestepped a query as to whether the two leaders had discussed attempting to jointly pressure Boeing by refusing to buy its planes.
Canada last month tried to end the dispute by suggesting it could withdraw a threat not to buy the Super Hornets if Boeing withdrew the challenge, sources said, but Boeing rejected the idea. (Additional reporting by Alwyn Scott; editing by G Crosse)