The British government has rejected a call to instigate a public inquiry into a botched “cash for ash” green energy scheme set to cost Stormont about £500 million.

It made clear its view that the Renewable Heat Incentive (RHI) controversy was a devolved issue for the Northern Executive to deal with.

Alliance party leader Naomi Long had written to Northern Secretary James Brokenshire and the chief secretary to the Treasury David Gauke demanding intervention from London on the furore.

In response, a government spokesman said: “The operation of the RHI scheme is a matter for the NI Executive. So it is right for the Executive and the Assembly to decide the form of any investigation or inquiry.”

The development comes amid an escalating political row that has edged the powersharing institutions towards implosion.

Ms Long had earlier said that, in any other democracy, Northern Ireland’s First Minister Arlene Foster would be gone over the “cash for ash” scandal.

“The only way we can move forward is to have a completely independent public inquiry and it would be wise for the First Minister to step aside without prejudice,” Ms Long told RTÉ’s Morning Ireland.

“How can the First Minister have public confidence after this?” she asked.

‘Not a normal democracy’

“But we are not in a normal democracy in Northern Ireland. It is one thing to say we have an unusual system in Stormont and it is another to say that politicians are above it,” she said.

“This is now a very difficult situation, it is a matter of great concern for the public.”

Ms Long said she did not understand why the DUP was taking such a “hard line”.

On Monday, Sinn Féin raised the prospect of the collapse of Stormont and Assembly elections being called if Ms Foster does not step aside as First Minister.

Party chairman Declan Kearney said that the political situation in Northern Ireland would be “entirely untenable” if Ms Foster does not stand down pending an investigation into the renewable heat incentive (RHI) scheme.

He said there was a “real prospect” of elections being called if Ms Foster did not stand aside.

Mr Kearney is the latest in a line of senior Sinn Féin leaders such as Gerry Adams and Martin McGuinness exerting pressure on Ms Foster and the DUP over the “cash-for-ash” green renewable heat fiasco that could result in an overspend of more than £400 million (€469 million) to be borne by the Northern Ireland taxpayer.

Stinging attack

Mr Kearney, in a stinging attack on the DUP, said the RHI crisis was dragging the political process to an “unprecedented tipping point”.

He said: “It’s increasingly obvious the DUP have lost the run of themselves within the Northern political institutions.”

Referring to the recent cutting of an Irish language promotions grant of £50,000 by DUP Minister Paul Givan, he said: “The announcement from the DUP Communities Minister that he was cutting funds for the Líofa Gaeltacht bursary scheme just before Christmas was yet another example of this.”

Mr Kearney said the DUP must decide whether or not it was committed to the “ongoing transformation of Northern society”.

He said: “This much is clear: it is not acceptable for it to continue opposing parity of esteem for the Irish language and Irish identity; promoting institutionalised bigotry; rejecting and ridiculing the need for reconciliation; and treating the legacy of the past as a new battlefield.”

On that issue Ms Long said she said the decision was sectarian and should be rescinded.

“It was a bad decision. It seems that it was taken in a fit of pique and should be rescinded. It was done in a petty fashion to have a push back at Sinn Féin,” she said.

“The Irish language is not in the ownership of Sinn Féin.”

Additional reporting: Press Association