Glasgow Airport chiefs last year commissioned infrastructure specialists Aecom, in partnership with Transport Scotland and Glasgow and Renfrewshire councils, to examine public transport options following the decision to axe the £210 million Glasgow Airport Rail Link in September 2009.
A list compiled by Aecom at the end of last year included everything from trams to bubble cars, but Transport Scotland was shocked in April when Aecom's final 12 options included recommendations for a train line to carry passengers direct from the airport to the centre of Glasgow.
Fearing a political and public backlash over the Aecom report, it is understood Transport Scotland opted to withhold the findings, and exasperated key stakeholders by stalling further research into costings.
Transport Scotland said the delay was caused by the report not yet being finalised.
The Scottish Government faced outrage from Glasgow business leaders and Labour-run Glasgow and Renfrewshire councils when it pulled the plug on the project on cost grounds four years ago, despite the fact preparatory work had already begun at Glasgow Central Station.
Stuart Patrick, Glasgow Chamber of Commerce chief executive, said the project had become party political, adding: "That's an awfully long time to have passed where we haven't even got to the point where the costed alternative options are in the public domain."
A Transport Scotland spokesman said it would "take forward a detailed appraisal study with a view to publishing a final report early next year". He added: "The detailed appraisal will consider the cost-benefit analysis of the shortlisted options."
Aecom declined to comment.