Chief executive Stewart Adams told MSPs a review had been launched to identify which operations at the loss-making airport were profitable and which were "a real drain on resource".

Giving evidence to the Scottish Parliament's rural economy and connectivity committee, he said: "The cost of passenger operations will be looked at.

"The passenger side of the business does not make money."

Glasgow Shettleston SNP MSP and committee member John Mason, who raised the issue, later tweeted: "Passenger side does not appear to be profitable. Perhaps airport could continue without passengers."

Ryanair is the South Ayrshire airport's sole passenger operator, almost all of whose passengers are holidaying Scots and few of them visitors.

Its passenger total increased in the year to March by 3.5 per cent to 702,000.

However, that compares to Glasgow Airport's 5.8 per cent growth to 9.9 million passengers in 2017, and Edinburgh Airport's 8.6 per cent rise to 13.4m.

Ryanair is Prestwick's sole passenger airline

Mr Adams said Prestwick had contacted 23 airlines to try to persuade them to fly from Prestwick.

He said those showing interest included a Cypriot airline, but it did not have sufficient pilots to launch flights this summer.

Mr Adams also said attracting London flights would be "very, very difficult" because of the number of flights from Glasgow, and the need for three or four a day from Prestwick to attract business travellers.

Prestwick was bought by the Scottish Government in 2013 for £1 to avert closure.

MSPs on the committee said the airport had since been loaned £40 million but had lost as much since the acquisition - £25m - as it had in the previous four years.

Scottish Liberal Democrat transport spokesman Mike Rumbles claimed taxpayers would never get their money back.

However, Scottish Conservative Ayr MSP John Scott said the value of the airport's 800-acre site as building land would surpass that.

He said: "If the business went belly up, the Scottish Government would still get its money back."

Prestwick chairman Andrew Miller said it had received several offers to buy the airport, but it did not want to sell the site for housing.

He also said there were "robust signs of growth", such as in military flights.

Property occupancy had also increased from 50 to 90 per cent.

Mr Miller said: "I firmly believe Prestwick can have a distinctive future as a multi-faceted centre.

"We will have a solution and a way forward by the end of the year."

Prestwick is also hoping to be chosen as a hub for construction materials for Heathrow airport's third runway, but it is competing with nine other Scottish sites.

Its bid to become the UK's spaceport for horizontal launches has also been delayed.

Scottish Conservatives transport spokesman and committee member Jamie Greene said later: “Despite receiving a staggering £40m taxpayer subsidy over six years, it is astonishing to learn the senior team at Prestwick has never once worked out the profitability or loss of its passenger operations.

“By their own admission, the prospects of funding new airline customers is looking increasingly bleak and distant in such a competitive market.

“Surely we are close to crunch time on deciding whether it is right to subsidise Ryanair’s commercial operations in this way with taxpayer’s money.

“There is a duty on the Prestwick executive board to admit the reality of the airport’s future prospects and focus on those parts of the business which might be profitable instead.”

Mr Rumbles said later: “I would not advise anybody to invest in Prestwick.

“The company is a dead duck and has lost money for nine straight years.

“When I pressed the committee witnesses on when the Scottish tax payer could expect to get this back, the directors had no answer.

“It’s quite clear that if they did ever ask for the money back the airport would go bust.”

The Scottish Government said the airport operated on a commercial basis at arm’s length, and ministers did not intervene in specific commercial discussions.

A spokesman said: “The Scottish Government wants Prestwick to continue to grow as an aviation facility with a long-term future.”

A spokeswoman for the airport said: “We are delighted we continue to be seen as key to Ryanair’s operations in Scotland.

“We continue to press hard to attract new operators and in the last year we approached 23 passenger airlines.

“This work continues apace but it is the management team’s firm view that success in turning Prestwick around will come with a broader approach to business development.

“Prestwick as a sustainable proposition points to focusing on its strengths as a niche airport, one less dominated by air passenger operations than rivals.

“Prestwick’s potential as a spaceport and the airport’s key role in Ayrshire’s bid to become a logistics hub for Heathrow’s expansion remain major development opportunities.”

Passenger flights at Prestwick Airport to end?