The inquiry into the delays, disruptions and spiralling cost of the Edinburgh tram project could take as long as two years to complete.

Witnesses are unlikely to be called to give evidence to Lord Hardie before the autumn, meaning the final report is not expected to be completed until well into next year.

That means any potential criticism of the role of the Scottish Government is likely to come after May's Scottish Parliament elections.

The Scottish Government has been accused of compounding problems by taking a "hands-off" approach to a project in which it had invested £500 million.

When the inquiry was announced by then-First Minister Alex Salmond, Scottish Government officials insisted it would be concluded "quickly and efficiently".

It was a claim that was reiterated when the inquiry's powers were upgraded so witnesses could be compelled to appear.

But the information-gathering phase of the inquiry is understood to be proving more complex than for previous similar investigations.

It is understood this is because the volume of electronic communication being stored has grown dramatically in the intervening years.

Critics yesterday described the inquiry's progress as "very disappointing" and one even claimed it risked becoming a fiasco to rival the trams themselves.

A source close to the tram project said: "It's become clear that no witnesses will be called to give evidence to the inquiry before the autumn.

"That suggests that the outcome will not be known until after the Scottish election next year which will be very convenient for the Scottish Government."

A council spokeswoman confirmed that chief executive Sue Bruce and transport convener Lesley Hinds had yet to be invited to give evidence.

The inquiry published a ten-point "order of events" in December, setting out the stages the inquiry would follow.

However, it gave no timescale for how long any of the stages would take to complete.

Its website states that it is currently on stages three and four, which involve "preliminary investigation" and "gathering material", which has been ongoing since December.

The inquiry has collected over two million digital files and 200 boxes of paper documents since Lord Hardie was put in charge in June last year.

Lothians Tory MSP Cameron Buchanan claimed the inquiry was unlikely to prove useful.

He said: "We have previously said that an inquiry into the Edinburgh trams would be fruitless - and we have been shown to be correct.

"This process is quickly becoming a fiasco in its own right and it seems that the only outcome will be a further deterioration in public confidence.

"Given that it seems unlikely that anyone who could have been held to account will be, it is now time to draw the matter to a close as quickly as possible and to focus on making the tram system work for the people of Edinburgh."

Transport consultant Robert Drysdale said: "My concern at the moment is that Lord Hardie will be calling for evidence from members of the public, but as far as I'm aware he hasn't yet done so.

"I don't know what size of a team he has sifting through all this - probably not big enough. It's very disappointing."

Mr Drysdale added that he had contacted the inquiry to suggest taking evidence from successful tram networks around the UK, but had not received a response.

SNP MSP Marco Biagi has previously criticised the inquiry for being "near comatose".

He said: "The danger is that members of the public lose interest in submitting evidence as the inquiry drags on."

A spokesman for the inquiry said: "The Edinburgh Tram Inquiry is making good progress in line with its published order of events and is at the stage of gathering material and retrieving and reviewing documents.

"Each stage is dependent on the previous and some of the stages may run concurrently."