Prospect, representing 3,500 air traffic controllers and engineers at NATS, said any part-sale of the airlines’ 42% stake would contradict the guarantee made by the Airline Group when it bought its stake that it would operate on a ‘not for commercial return’ basis.

Prospect General Secretary Paul Noon said: "Speculation that the Airline Group is considering selling its stake, just five years after the privatisation of NATS, underlines the concerns the union raised at the time of sell-off about safety standards and investment.

"If the Airline Group or parts of it wish to sell their stake then the government should buy it back. There must be no ‘For Sale’ sign over NATS.

"The purpose of NATS is to provide a safe service for the travelling public, not to be a cash cow for its owners. That is the basis on which the public-private partnership was established five years ago, backed up by guarantees to parliament."

Given the growth in air traffic and other major European initiatives, the UK air traffic management system needs stability and investment to provide a safe, efficient and integrated service, said Noon.

Prospect is writing to the Secretary of State for Transport, Douglas Alexander, raising its concerns. It wants an early meeting to seek assurances that the government will boost its 49% stake in NATS if necessary, and restate its commitment to continued investment in the business.