Scotland's spending watchdog is to urgently review the planned trams and airport rail link for Edinburgh.
The government will outline its position before the summer break
The Scottish government, which has concerns that the two projects do not represent value for money, has now called in Audit Scotland.
It will look into the procedures which were used to forecast costs.
The SNP administration will set out its position to parliament before the summer break, but Labour said the move was a further hold up to the projects.
Finance Secretary John Swinney said his priority was to protect Scottish tax payers and ensure that major transport projects benefited travellers and the economy.
"I want to be absolutely sure about the calculation of the costs involved in these projects, and the assessment of risk involved, before they progress any further," he said.
Audit Scotland, which has been asked to "review the process used to assess costs and risks to delivery", is to submit its report by 20 June.
The projects previously received parliamentary backing, although no final decisions have been taken on their future.
The SNP minority administration last week agreed to accept a cross-party amendment in parliament not to "arbitrarily" delay or cancel major transport projects in Edinburgh.
The Scottish Tories welcomed the move if it brought further clarity, but Scottish Labour transport spokesman, Des McNulty, said he did not support using the Auditor General to "overturn policy decisions".
Edinburgh Council welcomed the move saying the review would help in "formulating a prompt decision to end this period of uncertainty".
Councillor Phil Wheeler, who is an executive member for transport at the council, added: "It should provide the evidence and confidence for both politicians and the public to endorse the project going ahead.
"I am confident that the project has a rigorous governance and management structure which ensures there is a tight grasp on processes, costs and potential risks.
"Staff involved and contractors under consideration have UK and world-wide experience in delivering high quality light rail systems on time and budget."
But the Liberal Democrats' finance spokesman, Tavish Scott, said it was
unprecedented for an executive minister to order Audit Scotland to carry out an inquiry "purely to fix a political problem of their own making".
He added: "The SNP are not only trying to justify their own political bungle but in the process they threaten the independence and reputation of Audit Scotland."