The probe, examining the cost of the inquiry into the excessive overspending on the trams project, has so far hit a cost £3.7 million.

The inquiry was established in 2014 by then first minister, Alex Salmond, who said the review would be “swift and thorough”.

A spokesman for the Edinburgh Tram Inquiry said that the investigation was making “good progress”.

However it is still unclear when Lord Hardie, who is leading the inquiry, was due to report its findings.

The spokesman said: “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.”

The details of the costs of the inquiry were published by Transport Scotland in response to a freedom of information request.

It revealed that £1.822m has been spent on staffing costs, while legal fees have cost £716,000.

And the costs are likely to increase further as the Scottish government has allocated it a total of £2.5m for 2016-17.

The revelation of figures has led to numerous calls for the inquiry to be brought to a swift conclusion.

Miles Briggs, a Conservative MSP for the Lothians, said: “Edinburgh taxpayers are quite rightly angry that the Edinburgh Trams inquiry has gone on for so long and at such a cost to the taxpayer.

“It is essential that lessons are learnt from the inquiry and that the findings should be reported back as soon as possible so we can see what went wrong during the original project.

“Too often public infrastructure contracts are allowed to overrun with Scottish taxpayers being exposed to the increases in costs.”

25 people are employed by the inquiry, which is currently reviewing more than six million documents. However, the inquiry will not determine if anyone is financially or legally liable and will only examine why the project cost so much while it delivered less than planned.

The inquiry will be funded by 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.”