Garry Graham, National Secretary for Aviation, said: “We fear that the government is to announce a sell-off of its interest in NATS from 49% to 25% in the Pre-Budget Report.

“This proposed sell-off has nothing to do with supporting a safe and effective air traffic control system for the UK. It is merely a short-sighted attempt by government to plug the hole in its finances.

“At a time when the Airline Group is seeking to ‘cut and run’ by selling their own stake in NATS, the speculation surrounding a potential government sell-off introduces exactly the kind of instability we warned of at the time of privatisation.

“The reality is that our members working for NATS have served the airline industry and the travelling public well. We have one of the best safety records of any country despite UK airspace being some of the most congested and complex in the world. Delays remain at an historic low.

“If there is any truth in the speculation at all, our members will feel betrayed by the government.”

Graham said Prospect has written to ministers seeking an urgent meeting and will be writing to all MPs to raise its concerns.

“We said at the time of the part-privatisation in 2001 and still maintain that air traffic control is too important to be left to the vagaries and instabilities of the market. No other government in the world has gone down this route of privatisation.”