The inquiry, led by Lord Hardie, will unlikely call witnesses before the autumn which means the report is not expected to be published before well into 2016.

The Scottish Government has been criticised for taking a “hands off” approach to the trams project after it invested £500 million.

When former first minister Alex Salmond announced the inquiry, government officials said it would be completed “quickly and efficiently”.

This claim was repeated when the inquiry was made statutory – allowing it compel witnesses to appear.

However, its information-gathering phase is understood to be more complicated than a typical inquiry’s. This has been attributed to the sharp increase in electronic communication stored in the intervening years.

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

“That suggests the outcome will not be known until after the Scottish election next year, which will be very convenient for the Scottish government.”

A spokeswoman for the council confirmed that transport convener Lesley Hinds and chief executive Sue Bruce had not yet been asked to give evidence.

The inquiry produced an “order of events” last December, comprising the stages of the inquiry but did not include a timescale.

The website provides that the inquiry is currently at stages three and four - “preliminary investigation” and “gathering material”.

Lothians Conservative MSP Cameron Buchanan said: “This process is quickly becoming a fiasco in its own right and it seems 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 is Lord Hardie will be calling for evidence from 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.”

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.”