Airbus has finalised a deal to take over part of the Spirit AeroSystems operation in Belfast.

The deal was first announced in July last year as part of a wider break up of the Spirit business.

The transaction also includes Airbus work packages at the Spirit factory at Prestwick in Scotland.

However, the future of the Prestwick operation is uncertain with Airbus saying it will “assess its long-term strategy for the site as part of ongoing industrial planning”.

Airbus added that the Prestwick operation is being acquired to ensure continuity of production but will operate as an affiliate business.

Alan Perry from the GMB union said the Belfast sites should “remain as one identity” and that the deal “raises serious concerns over the future of more than 2,500 workers who don’t work on the Airbus contract”.

“This company which has been here for more than 150 years we will fight tooth and nail to protect and maintain jobs for future generations,” he said.

The union Unite described the move as an “unprecedented threat” to Northern Ireland aerospace.

A union spokesperson said: “Hundreds of highly skilled Spirit workers are facing an uncertain future. The UK government must now intervene to secure not just the workers but the future of Northern Ireland aerospace.”

“It cannot drop the ball and allow the collapse of Northern Ireland’s strategic and world-class aerospace sector.”

In Belfast, Airbus will take ownership of the A220 wing manufacturing and assembly facility.

The A220 mid-fuselage work package will also transfer to Airbus if a buyer for the remainder of the Belfast site is not found prior to closing of the deal later this year.

Airbus said: “These activities are critical to Airbus’ production ramp-up and will be stabilised within the Airbus operating system.

“Non-Airbus operations in Belfast will transfer to Boeing or may be divested to a third party by Spirit.”

Spirit is Northern Ireland’s most significant manufacturing employer, with about 3,500 staff.

The confirmation of the deal will still leave uncertainty for the staff who work in the non-Airbus part of the Belfast operation.

That part of the business mostly involves work on a range of Bombardier business jets.

Spirit’s wider global business is being carved up between Boeing and Airbus, the world’s largest aircraft manufacturers.

Spirt was founded in 2005 when part of Boeing’s business was spun off as a separate company.

The break up deal was initiated by Boeing which wants to bring Spirit back in house as part of its efforts to tackle manufacturing problems.

Aside from the Belfast operation Airbus will also take control of two Spirit factories in the US, one in France and one in Morocco.

The financial terms of the deal involve Spirit paying Airbus $439m (£329m) to reflect the fact the operations it is buying are currently loss making.

Airbus has also entered into a memorandum of agreement with Spirit to provide it with a $200m (£150m) loan which will be used to support Airbus programmes.