The chairman of a public inquiry into a botched green energy scheme that triggered the collapse of powersharing in Northern Ireland has vowed to conduct an objective and transparent investigation.

Retired judge Sir Patrick Coghlin told the opening of the renewable heat incentive (RHI) probe that evidence would be considered in a measured and independent manner.

He told the hearing at Parliament Buildings, Stormont, that the inquiry team has already examined one million pieces of documentary evidence.

The probe will investigate the design and operation of the ill-fated RHI scheme, an eco-friendly state subsidy initiative that left the Stormont administration facing a potential £700 million overspend bill.

Tory ally and Democratic Unionist Party (DUP) leader Arlene Foster’s role in the RHI scheme was at the heart of the row that brought down the Sinn Fein/DUP coalition government in Belfast.

NI powersharing talksDUP leader Arlene Foster will be called next year (PA)

Mrs Foster is expected to give evidence to the inquiry in the new year.

Delivering a brief statement at the start of the first oral evidence session, Sir Patrick said: “This inquiry was established in the wake of a media and political turmoil during which strongly worded allegations and repudiations were exchanged.

“The inquiry was set up to provide a measured, objective, independent and publicly transparent investigation of the facts in accordance with the terms of the reference and that is what this inquiry shall seek to do.”

The chair then passed over to the lead barrister for the inquiry to commence an opening statement that is expected to last for most of this week.

David Scoffield QC noted there was “precious little” in the scope of the inquiry that was “uncontentious”.

Mr Scoffield’s statement will address the context in which the inquiry was established; its terms of reference; the way in which it has conducted its work since it was established and the themes which have emerged from the evidence gathered so far.