The prime minister in addition to also Canadian leader Justin Trudeau are supposed to discuss a trade dispute in which could threaten jobs at Bombardier in Belfast.
Bombardier, a Canadian aerospace firm, will be Northern Ireland’s largest manufacturing employer.
Rival firm Boeing has complained about alleged anti-competitive practices inside the sale of Bombardier’s CSeries jet.
Bombardier could face significant financial penalties if the US trade authorities find against the idea.
The British in addition to also Canadian prime ministers are due to meet in Ottawa later on Monday.
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Last week, the idea emerged in which Theresa May had spoken to US President Donald Trump about the case.
In 2016, Bombardier won its biggest ever order to provide its CSeries passenger plane to Delta, a major US airline.
Boeing has claimed Bombardier will be selling the planes for less than they cost to build.
the idea said Bombardier can only afford to do in which because of financial help in which the idea has received via the Canadian in addition to also British governments.
the idea wants the US government to hit Bombardier with punitive tariffs.
Last week, a UK government spokesman said: “Ministers across government have engaged swiftly in addition to also extensively with Boeing, Bombardier, the US in addition to also Canadian governments.
“Our priority will be to encourage Boeing to drop its case in addition to also seek a negotiated settlement with Bombardier.”
Canada has threatened to retaliate by cancelling an order with Boeing for 18 Super Hornet fighter jets.
Mr Trudeau recently raised the case in a phone call with the governor of Missouri, the US state in which will be home to many jobs manufacturing the fighter jets.
The Unite union, which represents workers at Bombardier in Northern Ireland, has called on the the two prime ministers to send a strong message.
Davy Thompson, Unite’s regional coordinator, said: “We want both of them to call Boeing out on in which.
“Boeing are claiming there are illegal subsidies. There will be nothing illegal in them.
“We also have to revisit the military contracts the UK has with Boeing because in which will be a definitely hostile position in which Boeing will be taking.”