THE cost of Edinburgh’s controversial tram inquiry is rising at a rate of around £100,000 a month, according to the latest figures.

Officials have revealed the overall cost of the probe, which is currently holding public hearings, has now reached £7.2 million.

In September it stood at £6.9m – with the majority of the cash going towards staff costs and “professional fees”.

It comes as former tram boss David MacKay said Edinburgh could have become “the laughing stock of Scotland, if not the world” had disputes not been resolved during the beleaguered project.

Scottish Conservative MSP Miles Briggs said people had “understandably lost patience” with the inquiry and “deserve answers on why it is taking such a length of time and why it costing so much taxpayers’ money”.

He said: “Much like the trams project itself, the costs of this process seem to be spiralling out of control and local residents are increasingly concerned and angry.”

The tram inquiry was set up more than three years ago under Lord Hardie, the former lord advocate, but only started hearing oral evidence in September.

Figures released at that time under Freedom of Information laws showed costs jumped by almost £1 million in the space of three weeks as witnesses began giving evidence in person.

Edinburgh’s tram line opened in 2014, three years late, substantially reduced in scope and £231 million over budget.

Announcing an inquiry into what went wrong in 2014, then First Minister Alex Salmond promised it would be "swift and thorough".

More than six million documents are being examined as part of the probe, but it will not hold anybody legally responsible or financially liable.

A spokesman for Transport Scotland said Government ministers “have no role in the day-to-day running of this inquiry”, but added: “They want it to be efficient, cost effective and deliver clear recommendations for the planning and construction of any future major tram and light rail projects of a similar nature.”

Yesterday, former tram boss David MacKay told the inquiry a dispute over construction work on Princes Street was used as a tactic to bring Edinburgh “to its knees”.

Mr Mackay was chairman of Transport Initiatives Edinburgh (TIE) – the arms-length council firm which led the delivery of the trams – when workers downed tools for months in 2008.

He said contractors had an "insatiable appetite to get more money", adding: “Edinburgh was at a standstill. Retailers were going crazy, tourists were going crazy.”

But in a written submission, the former head of the construction consortium tasked with building the network accused TIE of being “lying, conniving and arrogant”.

Richard Walker, who was managing director of Bilfinger Berger UK, said the behaviour of TIE had been “utterly disgraceful”.

He added: “To call themselves public servants is an absolute disgrace.”

It is believed Lord Hardie will continue hearing oral evidence until at least the end of the year.

Evidence given to the inquiry will then be reviewed before an official report is compiled, which could take several months to complete.

A spokesman for the Edinburgh Tram Inquiry said: “All efforts are being made to ensure the Inquiry delivers best value for the public purse and we continue to make good progress in line with the published order of events.”

Edinburgh councillors agreed the outline business case for extending the existing tram line to Newhaven earlier this year, at an expected cost of £165m.

The final decision on whether to go ahead with the plan will be made in autumn next year.