Image copyright PA Image caption Martin McGuinness and Arlene Foster hold joint office in the roles of first and deputy first minister

Stormont's deputy first minister has called on Arlene Foster to "stand aside" as first minister while the 'cash-for-ash' scandal is investigated.

Mrs Foster set up the Renewable Heat Incentive (RHI) scheme in 2012 but an overspend could cost taxpayers £400m.

Martin McGuinness said he was concerned that "credibility of the political institutions is being undermined".

But Mrs Foster replied that she would not be stepping aside and "does not take her instructions from Sinn Féin".

'Corruption allegations'

Sinn Féin and the Democratic Unionist Party (DUP) are in a power-sharing coalition and they hold joint office in the roles of first and deputy first minister.

The statement from Mr McGuinness follows similar calls for Mrs Foster to step aside by the Ulster Unionist Party and the Social Democratic and Labour Party (SDLP).

Mrs Foster became leader of the Democratic Unionist Party (DUP) a year ago this week and has been first minister of Northern Ireland since January.

She was responsible for introducing the RHI scheme in her former role as minister for enterprise, trade and investment.

However, overgenerous offers of fuel subsidies and a lack of cost controls meant the scheme overspent by hundreds of millions of pounds.

Mr McGuinness said the RHI allegations included claims from former DUP minster Jonathan Bell that there was "corruption".

Mark Devenport, Political editor, Northern Ireland

Martin McGuinness' call on Arlene Foster to step aside has moved things on to a different level.

Sinn Féin and the DUP are now saying very different things.

Sinn Féin is probably under pressure and some of its supporters will be asking - why are you propping up these people?

They feel that they must be seen to be doing something.

Sinn Féin could side with the opposition in Monday's no confidence motion, which would be extraordinary, but it would be symbolic as it would need cross-community support in the assembly.

Mr Bell claimed some DUP special advisers (SPADs) attempted to "cleanse the record" by removing references to Mrs Foster and her department from documents linked to the scheme.

The ex-DUP minister also alleged some SPADs prevented him from closing down the most expensive part of the scheme last autumn.

Media playback is unsupported on your device Media caption RHI scandal: Bell claims DUP advisers 'interfered in closure'

"Taxpayers' money wasted in this scheme, needs to be retrieved," Mr McGuinness said.

"It is my belief the only way to establish the truth, and rebuild the reputation of the institutions, is to urgently establish a fully independent investigation into this matter.

"In addition, I also said that, in the public interest, she should stand aside from the role as first minister while that investigation is under way and at least until an initial assessment had been concluded into the veracity of all the allegations."

In response, a DUP statement said: "The first minister will not be stepping aside, but instead is focused on ensuring the full facts about this issue emerge and proposals are brought forward which can make a significant reduction in the future financial burden the Executive would face.

"The first minister does not take her instructions from Sinn Féin, but from the electorate."

The SDLP leader Colm Eastwood welcomed the Deputy First Minister's intervention.

He said: "It's right that he now moves to adopt the position of authority that Opposition parties have already taken.

"Sinn Féin must say publicly and clearly that they will support our efforts to restore confidence in our institutions and reach the truth of this matter. This is not a time for further equivocation."