The public inquiry into the £776 million tram project could take as long as to two years to deliver, it has emerged.
The scaled-back Edinburgh tram system launched in May this year after a five-year delay amid sky-rocketing costs and bitter internal disputes.
The two-year timescale was revealed at a meeting of the public spending watchdog Accounts Commission for Scotland last month.
The timetable exceeds the length of the 2004 inquiry into the controversial Scottish Parliament project, which took only 15 months to report its findings.
By the time it launched this year, the Edinburgh Trams project was £375 million over budget - with a dispute between TIE, the arms-length company in charge, and main contractor Bilfinger Berger at one point bringing work to a standstill.
Labour transport spokesman Mark Griffin said: “Serious questions need to be asked about the time this inquiry will take.
“When previous large-scale inquiries have taken far less time, the question has to be asked, why the delay in reporting? We need answers as to what went wrong and we need them quickly.”
A spokesman for the Edinburgh Trams Inquiry said: “There are a number of stages to the actual Inquiry, with each dependent on the previous.
“We are currently in the preliminary investigation stage, which includes retrieving and reviewing documents, scoping the work to be carried out and identifying potential witnesses, as well as securing sufficient staff and technical resources to manage the material.
“Only once this has been completed can a determination be made about when public hearings will be held. Lord Hardie will carry out the inquiry as efficiently as possible in a thorough manner, involving the examination of a large body of evidence.”
The non-statutory public inquiry into the tram project was launched by First Minister Alex Salmond in June and as being led by Lord Hardie, a former judge and Lord Advocate.
Last month it was revealed witnesses could be allowed to give evidence to the inquiry anonymously to encourage whistleblowers to come forward.