Passing of the torch: Eileen and Ian Paisley with Peter and Iris Robinson at a DUP rally to mark the end of Paisley's reign as party leader in May 2008

The Rev Ian Paisley is "beyond fury" following revelations about the behaviour of Peter and Iris Robinson, sources have told the Sunday Tribune.

The former DUP leader is deeply saddened by fears that the party he founded, and led for 37 years, could face heavy electoral losses if urgent action isn't taken.

It is understood that Paisley, as a family man, is appalled that Iris Robinson had an affair with a teenager she had known from the age of nine and who was in an emotionally vulnerable position following his father's death.

Meanwhile, it emerged this weekend that Iris Robinson also had an affair with 19-year-old Kirk McCambley's father, a butcher who died from cancer. She had another affair with a fellow DUP member in the 1980s which was witnessed by the security forces.

Paisley is understood to share the feelings of many people across Northern Ireland that the DUP vote could collapse in May's Westminster election. While the majority of DUP MPs still publicly support Peter Robinson, a growing number of senior colleagues have grave doubts about his future.

Willie Frazer of Fair, a group representing thousands of IRA victims, is also demanding Robinson resign. North Belfast MP Nigel Dodds is favourite to take over with East Derry MP Gregory Campbell as his deputy. However, it is unclear if Dodds wants the job.

"Peter must go and go as quickly as possible. He is damaging us all," a party officer said. One prominent politician stated: "The danger is that we will all be punished at the polls for the Robinsons' behaviour."

While not calling for Robinson's resignation, a veteran member said: "This is the lowest point in our party's history. It's a grim situation." Another senior DUP figure said: "The next few days will be vital in determining whether Peter can recover his position politically or whether he is regarded as an electoral liability."

There are calls for a full investigation into all the Robinsons' past dealings with two property developers, Ken Campbell and the now deceased Fred Fraser, who helped fund the business of Iris Robinson's lover.

Peter Robinson has sought legal opinion from a departmental barrister as to the correctness of his actions in not disclosing his wife's financial transaction to the appropriate authorities. Traditional Unionist Voice leader Jim Allister said this was wholly inadequate and demanded "a proper independent investigation".

While Paisley shares Iris Robinson's religious objections to homosexuality, he is said to be deeply disturbed that she was lecturing others while committing adultery.

Paisley was one of three DUP MPs who didn't issue a statement supporting Peter Robinson in advance of the BBC Spotlight programme. It is understood he believed MPs should have watched the programme and considered the allegations before giving Robinson their absolute backing.

Iris Robinson's affair with Kirk McCambley isn't her first. She had an affair with a fellow DUP member in the 1980s.

It was witnessed by security force members voluntarily guarding the Robinsons' house after the RUC officially withdrew their protection over Peter Robinson's loyalist invasion into Clontibret.

Fair director Willie Frazer, said: "Iris Robinson securing £50,000 from businessmen for her lover contrasts strongly with the DUP's lack of largesse for IRA victims.

"The widows of security-force members living on the poverty line are very angry. Peter Robinson must resign. It's time to rebuild the credibility of unionist politicians."

A Panorama programme tomorrow on the Robinsons will place further pressure on the DUP leader.