Witnesses will now be compelled to give evidence to the Edinburgh trams inquiry following its conversion from a non-statutory to full, statutory inquiry by the Scottish Government.

Lord Hardie, who heads the inquiry, was forced to ask the Scottish Government in recent weeks to intervene as some witnesses refused to co-operate and could not be forced to do so under the weaker, non-statutory form.

The inquiry’s remit is to investigate the trams debacle, which was years late and hundreds of millions of pounds over budget, in order to find out what went wrong and to prevent it from happening again.

Deputy first minister, Nicola Sturgeon announced the inquiry will now convert itself into a statutory one, meaning witnesses who refuse to cooperate will face prosecution.

The 14km tram line was intended to cost £375 million but could end up in the region of £1 billion following extreme overruns and delays.

Ms Sturgeon said on Friday that Lord Hardie had “reported a lack of co-operation by some, which is clearly unjustifiable”, though it is not known which parties refused to co-operate.

Ms Sturgeon said: “It was the view of the Scottish government that a non-statutory inquiry with the co-operation of those with knowledge of the project was the simplest way to ensure the swift answers that people want.

“Lord Hardie has, however, now reported a lack of co-operation by some, which is clearly unjustifiable. I have therefore given the inquiry the statutory powers he has requested to ensure that the necessary evidence is secured and a robust final report produced.

“Lord Hardie has assured me that converting the inquiry to a statutory basis will not increase the costs and time required, as he had intended to apply similar procedures. I continue to attach great importance to an inquiry that is quick, efficient and cost effective.”

The Inquiries Act (Scotland) 2005 provides that a person of interest who refuses to provide evidence could face up to six months in jail.

In a statement, Lord Hardie said: “The inquiry is currently in the preliminary investigation stage, which includes retrieving and reviewing a large body of documentary evidence, scoping the work to be carried out, identifying potential witnesses, as well as securing sufficient staff and technical resources to manage material recovered.”

Jim Eadie, SNP MSP for Edinburgh Southern, who wrote a parliamentary question on the issue, said:

“Given the widespread public anger over the mismanagement of the Edinburgh trams project, it is only right that Lord Hardie be given the extensive powers necessary to conduct a full and thorough investigation.

“By making the inquiry statutory, Lord Hardie will be able to compel witnesses to attend and insist information helpful to the inquiry will be made available.

“The Edinburgh trams debacle has become a byword for incompetence and mismanagement.

“It is vital the inquiry is able to unearth the facts, hold to account those that were responsible for the mistakes that were made and learn the lessons so that these mistakes are not repeated.”