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Sinn Féin has said terms of reference drawn up by the head of the civil service for an investigation into the RHI debacle are short of what is required.

First Minister Arlene Foster said the terms for an independent inquiry were worked out in consultation with the attorney general.

She said they were sent to Sinn Féin who, so far, had not responded.

However, Sinn Féin's MLA Conor Murphy said they were "insufficient".

"There has been no agreement with the DUP on the terms of reference of an independent investigation," he said.

"In order to restore public confidence in the political institutions we need an independent investigation, which is robust, transparent, timeframed and led by a senior judicial figure from outside the jurisdiction and with the power to compel witnesses and subpoena documents.

"Arlene Foster should step aside to facilitate that investigation pending a preliminary report."

Set up in November 2012, the Renewable Heat Incentive (RHI) scheme was an attempt by the Northern Ireland Executive to help to increase consumption of heat from renewable sources.

It is approximately £490m over budget as businesses were receiving more in subsidies than they were paying for renewable fuel and the scheme became heavily oversubscribed.