Opposition politicians said that the findings raise “serious questions” regarding the Scottish Government’s Transport Minister Stewart Stevenson’s reassurances that the project was “on track” until it was abruptly cancelled by Finance Secretary John Swinney last week. Further details about the behind-the-scenes efforts to deliver the doomed project are likely to add to speculation that the decision was not caused solely by Westminster-driven budget cuts, as the SNP government claimed.

The revelations come as Transport Scotland admitted to a hitherto undisclosed £17m rise in the project’s costs, amid a briefing war with other stakeholders over the cause of unforeseen costs and delays.

Transport Scotland’s attempt to pass on responsibility for the project to Network Rail came after it spent much of last year assessing the project it inherited from Strathclyde Partnership for Transport (SPT) before apparently deciding to wash its hands of the prestige venture.

Senior Transport Scotland sources have told the Sunday Herald that the move was resisted by the state-funded rail company, before being finally vetoed by the Office of Rail Regulation

The Glasgow-based transport agency then lined up the troubled airport link for the axe in response to Swinney’s requests for across-the-board efficiency savings. It subsequently refused to award any contracts for the project, infuriating interested parties and making it all but impossible for the project to be completed in time for the Commonwealth Games in 2014.

Despite these ructions, the transport agency, and Stevenson himself, were publicly insisting that GARL would go ahead and meet the Games deadline.

Patrick Harvey, convener of the Scottish Parliament’s transport, infrastructure and climate change committee and co-convenor of the Scottish Green Party, said: “[These revelations] mean that John Swinney and Stewart Stevenson have some pretty serious questions to answer before the committee, because MSPs have been told much more recently than Easter that everything was on track with the project. We will want to put them under the microscope and have them explain the position more clearly.”

Peter Hughes, chief executive of Scottish Engineering, criticised the government’s refusal to admit to any problems with the project in recent months.

He said: “The government hasn’t handled it particularly well given the lack of discussion about any problems over the past four or five months and the reassurances that were given that it would go ahead. It doesn’t make them look very clever.”

He added that the decision, which is estimated to cost 1300 jobs and many more indirectly, would be particularly damaging to the construction sector.

Des McNulty, Labour transport spokesman in Scotland, said the move left the SNP’s transport policy “in tatters”, as it meant they had cancelled both airport rail links for the country’s two main cities within two years. He also said that it raised “issues about the competence” of Transport Scotland.

Transport Scotland sources quickly moved to inculpate others for the project’s ultimate failure, claiming that SPT’s costings had vastly underestimated the true cost of the project, which was to link Glasgow airport to Central Station.

The agency claims that SPT costed the preparatory work required at the airport at £16m when it passed on the project last May – having estimated one year earlier that it would cost less than £8m. But Transport Scotland insists that this somehow overlooked obvious costs linked to the compensation of disrupted businesses and miscalculated cost of moving the airport’s “fuel farm”. The agency calculated that by the turn of the year, the work on the fuel depot would cost as much as £70m.

However, BAA sources ­challenge this figure, insisting that the highest estimate they ever received for such costs was £50m.

The most complex part of the project was to have been the part between the airport and Paisley Gilmour Street station – the so-called branch-line works – which involved building a bridge over playing fields. This is currently costed at £147m, or £17m more than the £90-£130m estimate being peddled by Transport Scotland until last week.

Decisions on the bridge and airport infrastructure components were repeatedly delayed throughout this year. In the airport’s case, BAA is understood to have selected Swedish construction group Skanska as its preferred bidder in June, but Stevenson and Transport Scotland refused to sign the contract, preventing work from going ahead.

For the airport-Paisley segment of track, Transport Scotland announced that “delivery of the new rail link had taken a major step closer” late last year and that it had earmarked a shortlist of four possible contracting consortia, but despite its claims, it effectively blocked progress by not moving to a formal tender during this year.

The agency insisted to the Sunday Herald that it held over both these processes pending the end of discussions with BAA over the airport. But BAA sources counter that there was ­“nothing of substance” that required to be discussed beyond the early part of this year.

The remaining part of the project is being carried out by Network Rail. This includes building two new platforms at Central Station and a third railway line between Paisley and Glasgow and providing signalling work. Transport Scotland said this work would still go ahead, despite claims that GARL’s cancellation makes it redundant.

On the subject of this year’s delays, Transport Scotland said: “Transport Scotland and BAA undertook detailed discussions in relation to the work in the airport campus. As GARL was a complicated project, it was necessary to assess each element properly prior to entering commercial commitments to ensure public money was best spent. Until these discussions had been concluded and all remaining uncertainties removed or addressed, Transport Scotland was not prepared to issue the formal tender for the branch-line works. Accordingly, no tender bids have been received by Transport Scotland. Similarly, Transport Scotland was not prepared to sanction the award of the contract for the fuel farm works, for which the tax-payer would have been liable under the terms of the master agreement between Transport Scotland and BAA.”

As GARL was one of the few capital projects on the government’s books, it has been seen as more vulnerable to budget restraints than flagship policies such as the council tax freeze and preserving university funding.