Boeing will not appeal its defeat in a US trade dispute with Bombardier, a case which once threatened large-scale job losses in Belfast.
The trade union Unite said Boeing's decision means "the runway is clear for take off" of the CSeries aircraft.
In January, the US International Trade Commission ruled in Bombardier's favour, overturning 292% import tariffs on sales of the plane in America.
Boeing, who brought the case, did not explain its decision not to appeal.
Wings for the CSeries are made in east Belfast and the tariffs - had they stood - would have hit sales and put 1,000 or more jobs at risk.
Unite's regional secretary Jackie Pollock said: "This means we have seen off the Boeing-inspired threat to jobs.
"This will come as very welcome news to the entire workforce at Bombardier in Northern Ireland."
The DUP MP for East Belfast, Gavin Robinson, said Boeing's decision "brings and end to this long and difficult saga".
He added: "Boeing should feel suitably chastened that their bullying approach was justly and robustly rejected."