Ministers have denied a buyer has been identified for Prestwick Airport as they put the debt-ridden site up for sale.
Announcement of the official attempt to off-load the South Ayrshire complex came despite several businesses expressing interest since it was saved from closure in 2013.
The airport was bought by the Scottish Government for a nominal £1, but it has since needed loans totalling nearly £40 million to stay in business and loses some £7m a year.
An aviation source told The Scotsman: “It seems an odd decision if there’s no one waiting in the wings.”
It is understood some firms involved with the airport could be potential buyers.
The move comes 18 months after speculation about a potential sale of the airport came to nothing.
A Scottish Government spokesperson said: “There have been a number of businesses and individuals interested in buying the airport since 2013.
“In light of that, we want to test the market and this process allows potential buyers to submit a detailed proposal.”
Transport Scotland said the loans totalled £39.9m at the end of March, with a further £7.4m available in the year to next March if required.
Another transport source said the size of loan could be causing concern.
He said: “The public spending watchdog Audit Scotland should be questioning whether this is a loan or a subsidy.
“You cannot give a loan indefinitely to a business which is not financially sustainable.”
In a previous audit of Transport Scotland, the watchdog said it “should continue to review the potential to recover the full value of support provided”.
An Audit Scotland spokesperson said: “We will be monitoring developments as part of our annual audit of Transport Scotland.”
Ryanair is Prestwick’s only passenger airline, but further income comes from cargo and military flights, and other operations.
A spokesperson for AGS Airports, which owns and operates Aberdeen and Glasgow airports, said: “We shared our concerns on Prestwick with the Scottish Government when it was first nationalised and have always been clear that there needs to be a commercial response.
“We would welcome any outcome that brings the ongoing public subsidy to an end and will watch the process with interest.”
Transport secretary Michael Matheson said: “Since the Scottish Government bought Prestwick Airport, we have been clear it is our intention to return the business to the private sector when the time is right.
“The senior management team at the airport has continued to engage with potential buyers and investors to discuss proposals for developing the business under new ownership.
“Good progress continues to be made by the airport to increase revenue, deliver operating efficiencies and pursue exciting opportunities for the future, including a spaceport.
“In light of that progress, the airport will shortly place an advert in the Official Journal of the European Union inviting expressions of interest.
“Any proposals submitted as a result of the advert would be considered carefully before any decision was taken to divest our shareholding in the airport or any part of the business.”
Ayr Conservative MSP John Scott said: “It is good news Prestwick Airport is now being actively marketed for sale.and I hope that a buyer can be secured at the earliest opportunity.
Despite the problems the airport has faced over recent years, and the need for it to be taken over by the government in 2013, I have always firmly believed it has the potential for a bright future.
“Prestwick has all the foundations for success; the longest commercial runway and parallel taxiway in Scotland, a reputation of being Britain’s only fog-free airport, its own dedicated railway station and a thriving aerospace campus.
“What it now needs is an owner prepared to put in the investment to take the airport forward as the major economic asset it undoubtedly is.”
But Scottish Liberal Democrat transport spokesperson Mike Rumbles MSP said: “The purchase of Prestwick has been an absolute disaster for the Scottish Government.
“It has lost taxpayers’ money hand over fist. The government have put forward a series of over-optimistic proposals for Prestwick and then failed to deliver.”“I hope the site can be put to good use by its new owner and as much taxpayer’s money as possible can be recouped by this sale.”
Scottish Labour transport spokesperson Colin Smyth said: “The [Scottish] government were far too slow in making the changes needed to turn Prestwick around when they bought it and as a result it’s doubtful they will get back the money they have put in.“However, the Government cannot simply wash its hands of Prestwick.
“They have a responsibility to ensure that if a buyer is actually found, any sale must secure the jobs at Prestwick.”“Anything else would be an economic tsunami for Ayrshire and beyond.”