The scale of the Edinburgh Tram Inquiry became clear today as the first hearing in the probe revealed 500 million potentially relevant documents had been whittled down to six million now identified for examination.

Lord Hardie opened the preliminary inquiry aimed at providing an update on progress so far and identifying those expected to have a significant role in the investigation that was called by former First Minister Alex Salmond.

He said the inquiry team has narrowed the initial massive total of potential initial evidence sources down to six million documents that will require analysis as he named the core participants in the inquiry.

Edinburgh City Council, contractors Bilfinger UK, funder Scottish Ministers, contractor Siemens Plc, law firm DLA Piper, engineer Parsons Brinckerhoff, utilities contractor Carillion were named as being included as core participants in the inquiry.

The public hearing in Edinburgh was attended by about 20 members of the public.

It is claimed the Edinburgh Trams project cost businesses hundreds of thousands of pounds and forced some to close as the Edinburgh City Council project that was mostly led by the arms-length Transport Initiatives Edinburgh (Tie) ground to a halt mid construction because of a fall-out with contractors.

Lord Hardie made an opening statement revealing the progress of the inquiry and set out his process for conducting the next stage, but did not take detailed evidence from witnesses at this point.

The start of the inquiry comes after a postponement in August when inquiry chairman Lord Hardie was taken to hospital.

A spokesman for the 69-year-old former Lord Advocate said at the time the work of Lord Hardie’s inquiry team had continued during that time "in line with the published order of events".

The trams started running in 2014 after six years of disruption in the Scottish capital.

The project cost £776 million. The original cost was £545m and the first trams arrived three years late.