Spirit AeroSystems and Boeing are close to reaching a funding agreement that would give a cash lifeline to the struggling Boeing supplier, an industry source familiar with the matter said.

Aircraft-parts supplier Spirit AeroSystems Holdings Inc. on Tuesday said that there was “substantial doubt” about its ability to keep running as a “going concern,” as losses, debt, and difficulties at jet maker Boeing Co.

Boeing and the aerospace supply chain have navigated a costly labor strike with surprising ease. One of Boeing’s key suppliers, however, has run into significant trouble. Tuesday, fuselage supplier Spirit AeroSystems filed its quarterly report with the Securities and Exchange Commission.

According to a recent earnings report, Spirit ended Q3 with just $218 million cash on hand. The company previously announced a $425 million advance from Boeing, but that remains unpaid. The company also secured a $350 million bridge loan, and by the end of Q3, the entire amount had been borrowed.

Spirit AeroSystems, a key supplier to US planemaker Boeing, warned on Tuesday there was “substantial doubt” that the company would be able to continue as a going concern.

The designer and manufacturer for commercial and defense aircraft said its liquidity has been affected in part by the strike at Boeing and limitations on that company’s ability to increase production.

"We retain a range of options and levers to address our financial and spacing constraints and are working with our customers on these matters," a Spirit spokesperson said.

Spirit AeroSystems is warning that its business viability remains in doubt, saying its cash reserves are drying up due to Boeing's sharply reduced production rates and to other factors driving steep losses on its Airbus and Boeing programme work.

Wichita-based Spirit AeroSystems says it will soon end the furloughs of some of its employees and has no immediate plans to implement additional furloughs or layoffs after Boeing workers approved a contract offer on Monday.