A preliminary US finding on a trade dispute which could threaten thousands of Northern Irish jobs is due later.
Aerospace giant Boeing claims its Canadian rivals Bombardier received unfair state subsidies from the UK and Canada, allowing them to sell airliners at below cost prices to the US.
Bombardier employs around 5,000 people at a plant in East Belfast and signed a 5.6 billion US dollar deal last year to provide up to 125 of its new C-Series aircraft to Atlanta-based Delta Airlines. The wings are made in Belfast and Prime Minister Theresa May has lobbied President Donald Trump over the importance of preserving the manufacturing posts.
The US Department of Commerce could introduce a preliminary tariff on the C-Series, which would increase the cost of importing the plane to the US, when it publishes its initial findings later.
A recent Bombardier statement said: "It is pure hypocrisy for Boeing to say that the C-Series launch pricing is a 'violation of global trade law' when Boeing does the same for its new aircraft."
The Northern Ireland powersharing Executive and UK Government pledged to invest almost £135 million in the establishment of the C-Series manufacturing site in Belfast.
The programme received one billion US dollars from the provincial government in Quebec in 2015 when its fortunes appeared to be ailing.
Mrs May's intervention came after she was pressed to raise the issue by DUP leader Arlene Foster, whose 10 MPs are propping up her minority administration in the House of Commons and are expected to play a crucial role during upcoming Brexit business in Parliament.
The C-Series is a single aisle, mid-range aircraft which will provide competition for the Boeing 737.
Deliveries are due to begin next year.
The innovative carbon-fibre composite technology, developed in Belfast to produce the wings, enables material and aircraft weight savings, contributing significantly to the C-Series airliner's economic and environmental credentials, Bombardier said.
Delta is taking the smaller CS100 model of the plane.
Bombardier is a Montreal-based multinational which had been under financial pressure and invested large sums in its new jet programme. It is crucial to many jobs in Belfast.
In February last year the firm announced 1,080 job losses at its Northern Ireland plants following delays to the C-Series.
Bombardier is one of Northern Ireland's largest employers and supports hundreds of other jobs through its suppliers.