Unions are hopeful new owners can be found for Scotland's fourth-largest airport to arrest the decline that has been engulfing it for several years.

Glasgow Prestwick Airport is now on the market after its New Zealand-based owner said it was under-performing following a significant drop in passenger numbers, down around a million in just under five years.

Ryanair has ruled itself out of acquiring its first airport but said it was open to discussions with any prospective bidder or buyer, while Virgin Atlantic yesterday dismissed rumours it was in the running to take over Glasgow Prestwick.

But Unite, which represents non-air traffic control staff, said there would be little surprise at the news among the workers, even if owners Infratil had not informed them yet of the move.

The union said there had been a tangible sense of decline at the Ayrshire airport for a number of years and while there would be uncertainty about the future there would also be hope of new investment.

Paul Neilson, regional organiser for Unite, said there was little room for making further staff cuts at Glasgow Prestwick, with things already to the bone.

He said: "The Prestwick staff need some hope about the place, for the future of the airport and some job security even for the medium-term. The sale might just be the starting place for that. If there's no buyer there'll be concerns about where Prestwick will be in just a couple of years."

A Ryanair spokesman said: "We've said recently we're not interested in buying airports and that's the same deal with Prestwick.

"We've worked very well at Prestwick and would work on with any new owner to grow the airport and agree growth proposals if they want to talk to us before the sales process is completed."

One aviation insider said: "It's been well known within the industry that Infratil has been looking for a buyer for months but has just made that official now.

"With the uncertainty over Edinburgh [which is also up for sale] its hard to see who a buyer would be or indeed what the medium to long-term future is for Prestwick."

Yesterday, John Scott, the Conservative MSP for Ayr, expressed fears over job losses, adding "many families will be extremely worried about this".

Glasgow Prestwick chief executive Iain Cochrane said the decision to sell Prestwick was announced at the Infratil Investor Day in New Zealand, where the company also said it would sell Manston Airport in Kent.

He added: "This decision comes from a re-focusing of Infratil's investment profile and has been under consideration for some time. It's consistent with Infratil Chief Executive Marco Bogoievski's public comments over recent months.

"Prestwick is a great airport with a great team and a great future. I believe this is an excellent opportunity for us to attract new investment into the airport to provide the stimulus for future growth.

"Today, it's business as usual as the busy summer season approaches and we're totally focused on looking after our customers."