Northern Ireland's first minister, Peter Robinson, was fighting for his politcal life tonight over claims that he failed to inform the authorities about a £50,000 loan his wife passed on to her then 19-year-old boyfriend.

Robinson said he had agreed to an independent inquiry into his actions by a QC appointed by Stormont after calls from within the power sharing coalition for him to consider his position.

In a statement tonight he said he would fight "smears" against his personal integrity, adding that he had been "tried in the media and the press".

Castlereagh borough council, which granted Iris Robinson's lover permission to open a riverside cafe, confirmed yesterday that it was launching an inquiry into her role in enabling Kirk McCambley to open the Lock Keepers Inn on the southern edge of Belfast.

The main allegations against the Robinsons include:

• That Iris Robinson gained £50,000 for McCambley from two major property developers, Ken Campbell and Fred Fraser.

• That Iris Robinson lobbied on behalf of Campbell for a building scheme he was involved with in her Strangford constituency.

• That she took £5,000 of the money for herself.

• That when Peter Robinson found out about the loan and his wife's affair he insisted she pay the money back, but did not inform the authorities about her conduct.

• That when her relationship with the young man broke down she demanded the loan back and wanted £25,0000 paid into the account of her church, the Light and Life Free Methodist church in east Belfast.

Inside Northern Ireland's fragile power sharing coalition the nationalist SDLP became the first party to openly question if Peter Robinson could remain in his post. Mark Durkan, the outgoing SDLP leader, described the first minister's position as "questionable" after claims on BBC Northern Ireland's Spotlight programme about the scandal.

Durkan also challenged Iris Robinson to immediately resign from her jobs as Castlereagh borough councillor, assembly member and MP for Strangford.

The Foyle MP said: "Following the broadcast of last night's BBC Northern Ireland Spotlight programme, it is clear that Iris Robinson should stand down from her public roles forthwith.

"Serious questions must now be asked of Peter Robinson and it will be up to him to prove that the issues raised do not affect his ability or suitability to carry out his public office. If Peter Robinson is to be brave it is imperative that he calls for proper investigations to take place. Otherwise it would appear his position as First Minister is questionable."

The political ceasefire over the personal and financial troubles of Northern Ireland's "first family" first broke this morning with opponents initially calling for an inquiry into fresh allegations against the Robinsons. Twenty-four hours after an outpouring of sympathy for Robinson over the revelation that his wife had an affair, he now faces demands to explain allegations that he did not inform the authorities about the £50,000 loan to her teenage lover.

The Ulster Unionist party and the centrist Alliance party said he had questions to answer over claims that he had insisted that his wife repay the money but had not informed the authorities about it.

Sir Reg Empey, the UUP leader, has called for a public investigation into his conduct. "It is perfectly clear there are going to have be a number of investigations by different authorities into this," Empey said today.

Junior minister and Sinn Féin assembly member Gerry Kelly said Peter Robinson should convene an emergency meeting with Martin McGuinness, his deputy at the head of Northern Ireland's government, to discuss the latest allegations.

In his statement, Robinson revealed that the office of the first and deputy first minister would appoint an independent senior counsel to investigate if he had done anything wrong in failing to report his wife's loan. "I have indicated that I have done nothing wrong," he said.

Asked if he would take legal action against the BBC programme that levelled the allegations against him and his wife, he said: "I can't wait for one or two years to clear my name." He said that was why he had agreed to an inquiry by a lawyer appointed by the government.

On allegations that he had not called an ambulance for his wife after her failed suicide attempt on 1 March last year, but had left her to go to the assembly, Robinson said he had acted on medical advice that she should sleep after her failed suicide attempt.

"If I find out on the basis of that [legal] advice that I should have [reported the loan] then clearly this has consequences," he said. "Everything I have done I have done in a proper way. I am putting this to the test. I am handing my future to someone to look over these matters in an independent matter."