DUP leader Arlene Foster cannot return to the Northern Executive until an investigation into the cash-for-ash debacle is completed, Sinn Féin’s Northern Ireland leader has insisted.

Michelle O’Neill on Wednesday held to her party’s demand that Ms Foster cannot be re-elected as first minister – or deputy first minister if the DUP does not re-emerge as the largest party – until a public inquiry into the Renewable Heat Incentive (RHI) scheme is completed.

Speaking at the launch of Sinn Féin’s Northern Assembly election manifesto in Armagh, Ms O’Neill said: “While there is a cloud over Arlene Foster we will not be supporting her position for first minister or deputy first minister in the Executive Office.

“We have to have a full investigation. We have public concern and outrage and demands for answers. The RHI scheme has been all of the DUP’s making.”

Ms O’Neill’s position on Ms Foster has raised questions about how long Stormont will remain in mothballs, or if, power sharing can be restored.

Last month Sinn Féin Minister for Finance Mairtín Ó Muilleoir appointed retired judge Sir Patrick Coghlin to chair a public inquiry into the so-called cash-for-ash scheme which could result in an overspend of almost £500 million (€589 million) up to 2036.

Full support

At the time Mr Ó Muilleoir suggested that six months might be a reasonable period in which to expect publication of the report but other Sinn Féin sources suggested it would take longer. So far, the DUP is insisting that Ms Foster has its full support and must be in a position to be returned to the Executive Office.

If the parties maintain their positions then Stormont could be suspended for more than six months, with some form of British direct rule put in its place.

Ms O’Neill dismissed attempts by Ms Foster and other DUP politicians to portray her as a “puppet” of Sinn Féin president Gerry Adams since she took over as Northern leader from Martin McGuinness.

“I know what my job is as leader. I intend to lead my party into this election and into the future on the basis of equality, on the basis of delivering in terms of integrity and respect.”

Ms O’Neill repeated Sinn Féin’s demand for an Irish language Act but refused to describe it as a “red line” issue. While Ms Foster this week indicated that she could be prepared to see an end to the Assembly petition of concern, Ms O’Neill wanted it to remain in place.

The petition is a Stormont mechanism where with 30 or more MLA signatures motions can be vetoed even if they have majority support in the Assembly chamber. It was used by the DUP to halt a motion calling for same-sex marriage even though a narrow majority voted in favour of changing the law.

Ms O’Neill suggested that Ms Foster appeared to change her position in order to deflect attention away from the RHI fiasco. She said the petition of concern should be used for what it was originally intended, “to protect minority rights”.

She said the Northern Ireland Secretary James Brokenshire should not chair talks that are expected to take place after the election to try to restore the Executive and Assembly.

United Ireland

His position on issues such as Brexit and dealing with the past demonstrated that he was “not an honest broker”, she said.

Ms O’Neill also called for a referendum in the North and Republic on a united Ireland. “We need a Border poll and we need to see it now.”

She said Sinn Féin had “called time on the arrogance and contempt of the DUP” and that the party was showing “zero tolerance for racism, sexism and homophobia”.

Outgoing East Derry MLA Caoimhe Archibald said the 56 per cent majority vote in Northern Ireland to remain in the European Union must be respected. She said Brexit undermined the 1998 Belfast Agreement and that the North must be allowed “designed special status” within the EU.

Sinn Féin vice-president Mary Lou McDonald, who chaired the manifesto launch, said her party had taken a resolute stand against corruption and cronyism, and all issues that prevented government “delivering fully for all of the people”.

With the Fine Gael-led minority Government in the Republic facing a confidence vote on Wednesday night, Ms McDonald said the Republic was experiencing “some of the same behaviours that brought the administration down in Stormont”.

“Enda Kenny and his crew are put on notice that Sinn Féin are not for looking the other way,” she said.

Sinn Féin manifesto main points

• Referendum on united Ireland.

• Introduction of Irish language Act.

• Same-sex marriage legislation.

• 50,000 new jobs during lifetime of Assembly.

• Extra £1 billion for health.

• Build 10,000 new social and affordable homes.

• Extra £500 million for welfare recipients.

• £6 billion to improve roads, transport and public services.

• Increase to £525 million spending on childcare and early development.

• Ring-fence funding for front line policing.