The Scottish Government's "strategic vision document" on the future of the loss-making Ayrshire airport had been due to be published over the summer but will not now be released until October, two months behind schedule.
Nicola Sturgeon had told the Scottish Parliament's infrastructure committee on June 18 she hoped to publish the document "within the next couple of months".
James Kelly, Labour shadow cabinet secretary for infrastructure, said: "It is unacceptable that Nicola Sturgeon has still not published the SNP Government's plans for Prestwick Airport, two months after her commitment to a parliamentary committee.
"The public and the aviation industry are entitled to sight of the Government's plans. With £15 million of public money invested in Prestwick, a clear understanding is required of how the money is being invested and what is required to make Prestwick a sustainable business."
The strategic vision paper is based on the findings of a £100,000 taxpayer-funded report by finance consultant Romain Py, who submitted his report to the Government in mid-May. Mr Py's full report has not been made public on the grounds it contains commercially sensitive data that could jeopardise Prestwick's recovery if disclosed to rivals.
A spokesman for Transport Scotland said "further modelling work" on the business case is yet to be completed, but it will include a focus on the likely impact of a reduction or abolition of Air Passenger Duty (APD). The Scottish Government has vowed to eventually axe APD under independence to attract airlines into setting up new routes from Scotland.
The apparent delay in publishing the document comes weeks after Prestwick faced a fresh blow when its only carrier, Ryanair, halved its winter schedule and announced it was launching flights from Glasgow airport from October.
Alex Johnstone, Conservative spokesman on infrastructure, said he sympathised with the pressure ministers were under but urged them to publish the document as soon as possible.
He said: "They were foolish to make promises against a backdrop that they can't control because it just means you let a lot of people down. But sometimes you just have to be brave and they must stop dragging their heels over this. It may be that this report leads to another report if conditions change, but the whole of Scotland can't stay on hold for September 18. It's time the Scottish Government started acting more like a government and less like a referendum campaign."
A spokesman for Transport Scotland said: "A document will be published in October, setting out a strategic vision for the long-term future of the airport, investment plans, business development and the optimum operating structure required to take the airport forward."