Northern Ireland’s first minister, Arlene Foster, should step down immediately to avoid causing potentially lasting damage to the integrity of the devolved assembly, the leader of another party has said ahead of a no-confidence vote.

Mike Nesbitt from the Ulster Unionist party (UUP) said Foster’s role in a failed green fuels subsidy scheme that could cost the government £400m meant she had to resign.

“If the principle of ministerial responsibility is to have any meaning at all in Northern Ireland, she has to resign and take responsibility for what was an utterly incompetent, inept and hapless scheme,” he told BBC Radio 4’s Today programme.

The UUP, which has 16 seats in the Northern Ireland assembly compared with the 38 held by Foster’s Democratic Unionist party (DUP), has joined a cross-party effort to seek the first minister’s resignation over the fuel subsidy scheme.

A motion of no confidence in her leadership is being debated on Monday. It was tabled by the nationalist Social Democratic and Labour party (SDLP), with support from the UUP, Alliance party, Green party, Traditional Unionist Voice and People Before Profit Alliance.

Martin McGuinness, the deputy first minister, has called on Foster to step aside amid an investigation into the scheme, which offered financial incentives to farms, businesses and other non-domestic consumers to use biomass boilers that mostly burned wood pellets, as well as solar thermal and heat pumps.

Nesbitt said the flaws in the scheme meant the Northern Ireland government would pay people £160 in return for burning £100 of fuel and Foster had failed to take proper action after a whistleblower pointed this out.

Further inaction risked casting doubt on the integrity of the devolved administration, he said.

“I think we’ve got to the point where among the public in Northern Ireland, the reputation of the Stormont government is in the gutter,” Nesbitt said. “These institutions were very hard fought for 18 years ago and they were responsible for ending wholesale violence on our streets, and the public are in a position where they rightfully demand better.

“We’re at a point where now where there’s a decision to be made as to what is important, Mrs Foster’s career or the integrity of the institutions.”

The renewable heat incentive (RHI) was set up in November 2012 to encourage the consumption of heat from renewable sources in the region. In February, a whistleblower claimed that it was being abused and one farmer had made £1m from renting out an empty shed.

Northern Ireland’s auditor general, Kieran Donnelly, concluded this summer that there was “no upper limit on the amount of energy that would be paid for. The more heat that is generated, the more is paid”.

Last week, the former trade minister Jonathan Bell accused fellow DUP members of delaying the closure of the controversial scheme.

Bell also alleged that DUP advisers had tried to “cleanse the record” of any links between Foster and the decision-making process that led to the RHI’s creation. The first minister has apologised for not ensuring there was a cap on the millions being paid out to those who took up the scheme and made considerable profits from it.

On Sunday, the DUP assembly member for Lagan Valley, Jeffrey Donaldson, confirmed that Bell had been suspended from the party over his allegations.