The future of Glasgow Prestwick Airport would be aided by the devolution of air passenger duty (APD) to Scotland, the First Minister has said.

Alex Salmond pledged to set out a strategic vision for the airport in the next few weeks, before the end of his term in office.

Aberdeen, Edinburgh and Glasgow airports have already made a joint call for control of APD to move to Holyrood, claiming the tax is damaging Scotland.

They set out their case in a submission to the Smith Commission, which is considering further devolution for Scotland.

The Scottish Government bought Glasgow Prestwick last year for £1, taking it into public ownership after owners Infratil had earmarked it for closure.

Mr Salmond came under pressure over its future from Conservative leader Ruth Davidson, who said promises to outline its commercial future had been broken, with several reports on the matter delayed.

Ms Davidson demanded a "clear and unambiguous" route map for the airport during First Minister's Questions at Holyrood.

"After more than a year of uncertainty we do need this sorted for the long-term now," she said.

Mr Salmond replied: "That strategic vision will be published in the next few days. I can promise her within my remaining term of office.

"It will set forward an exciting future for Prestwick Airport that would be considerably assisted if Prestwick Airport didn't have to exist with its hands tied behind its back on the imposition of the outrageous duty of air passenger duty and its impact on the carriers at Prestwick Airport."

He said APD had been the "straw that broke the camel's back" in relation to a private-sector takeover of the airport.

The First Minister said: "The Government stepped in because the last private-sector bidder was unable to carry forward, and of course the significant thing that broke the deal and made it unable to go forward with that private-sector bidder was the impact of air passenger duty in terms of the flights from Prestwick."

He said he had spoken to Sir Howard Davies, who chairs the independent Airports Commission tasked with looking at the need for additional UK airport capacity and recommending to the UK Government how it can be met.

Mr Salmond said: "I put it to Howard Davies if you were to reduce APD for the north of England or give the Scottish Parliament the power to do something about it to increase the competitive position for direct international flights, then you would relieve immediately some of the pressure on the London airports.

"Howard Davies looked at me and said that would be a distortion of competition.

"Unfortunately Ruth Davidson and her party live in a world where spending £40, £50, £60 billion on infrastructure in the south of England is not a distortion of competition but allowing airports like Prestwick to survive and prosper by having a competitive rate of APD somehow is.

"That's the topsy-turvy world of London bias that the Conservative Party have placed on Scotland."