The investigation into what went wrong with the £776m project was set up by the then First Minister Alex Salmond in 2014, who said it would be “swift and thorough”.

A spokesman for the Edinburgh Tram Inquiry, which is being chaired by retired judge Lord Hardie, said it was making “good progress” but he was unable to say when it would report.

The inquiry has yet to hold any public evidence-taking hearings and staff are currently said to be reviewing over six million documents connected to the project.

The spokesman told BBC Scotland: “The Scottish ministers have been clear from the start that the inquiry should be timely and cost effective.

“Lord Hardie’s priority is to conduct a thorough investigation into the Edinburgh tram project and produce a report and recommendations that ensure lessons are learned for future major infrastructure projects.”

Figures published by Transport Scotland in response to a Freedom of Information request show £1.822m has been spent so far on staffing costs, £716,000 on legal fees, £540,000 on IT costs and £502,000 on accommodation.

A total of 25 people are employed by the inquiry.

The inquiry will not determine why the project went ahead, nor will it decide if anybody was legally responsible or financially liable.

It will examine why the project cost so much while delivering less than had initially been planned.

The Scottish Government has agreed to fund the costs of the trams inquiry until it is completed.

A spokesman for the government said: “We have been clear from the outset that the inquiry should be efficient and cost effective, however, we know that major infrastructure projects do generate large quantities of documentation, and we understand the inquiry team has already gathered over six million documents.

“Lord Hardie and his team is firmly committed to discerning the facts in a robust and thorough manner in order to ensure the final report is fair, balanced and offers clear recommendations for the planning and construction of future projects.”

The tram project was originally to extend to three main lines and was estimated in 2003 would cost a total of £375m to complete, with two of the three lines expected to be running by the summer of 2009.

However the link between Granton and Haymarket was shelved, as was a proposed line to the south of the city, as costs spiralled.

The one line tram system, running 8.7 miles from Edinburgh Airport to York Place in the city centre, opened in 2014, five years late and around £400m over the original budget.