The threat of renewed sectarian violence is at its highest in years. But it’s a seamy affair between Northern Ireland’s most famous female politician and a man nearly 40 years her junior that has put the province’s fragile peace pact between Roman Catholics and Protestants in danger of unraveling.

The woman in question is Iris Robinson, 60, the glamorous wife of the leader of Northern Ireland and a canny lawmaker in her own right. For months, she maintained a sexual relationship with a 19-year-old, then allegedly helped set him up in business with money secretly lent her by a pair of property developers.

Revelation of the affair last week was enough to trigger outrage in Northern Ireland, where Robinson had been an outspoken advocate of traditional family values and publicly denounced homosexuality as an abomination.

But allegations that she committed not just adultery but also financial malfeasance have thrown into doubt the historic power-sharing agreement between republican-minded Catholics and Protestants loyal to the British crown.

Robinson’s husband, Peter, is accused of having found out about the secret loans but not reporting them. That could force his resignation as Northern Ireland’s first minister at a crucial moment when many observers consider him one of the key forces holding the unity government together. The government is grappling with the contentious issue of devolving authority over police and the judicial system from the British government in London to provincial officials in Belfast.

Clues to Peter Robinson’s fate may emerge today after he meets with officers of his Democratic Unionist Party. Iris Robinson reportedly is already preparing to step down as a member of the British Parliament and of the Northern Ireland Assembly, her highflying political career almost certainly over.

David Trimble, a former first minister who shared in the 1998 Nobel Peace Prize for helping end the conflict in Northern Ireland, said Peter Robinson’s credibility had been fatally compromised.

“It’s clear from events within the Democratic Unionist Party that his authority has gone and his capacity to lead that party is very . . . seriously hindered,” Trimble told the BBC on Sunday. “I think that political reality over the next few days will lead the party toward a change.”

A change in leadership might precipitate elections in the province, which could then strengthen harder-line parties on both sides of the divide, putting an agreement on the hand-over of police and justice powers further out of reach.

The transfer of such authority is arguably the most important challenge still outstanding for the 1998 Good Friday peace accord that formally ended “the Troubles” in Northern Ireland. Republicans regard a hand-over as the necessary end to an instrument of British oppression, while unionists fear it would give their onetime bitter enemies the upper hand.

An agreement is already past its formal deadline, and the power-sharing government in Belfast, the Northern Irish capital, is teetering on collapse. Peter Robinson, though not highly popular as first minister, had been seen by many as one of the few leaders able to help steer an agreement through.

The public rallied to his side last week when, looking numb, he publicly revealed his wife’s affair after having kept it private for months.

But a few days later, the BBC broadcast an investigation alleging that Robinson knew his wife had solicited loans from businesspeople to help her paramour open a coffee shop yet failed to report it to authorities, as political ethics rules demand.

In a statement, Robinson declared that he had “done nothing wrong” and that he was being “tried in the media.” He promised to appoint an independent counsel to investigate the scandal.

But public sympathy for Northern Ireland’s “first couple” has now given way to anger -- and ridicule -- as the details of Iris Robinson’s relationship with Kirk McCambley, now 21, have leaked out.

She took McCambley, the son of her butcher, under her wing when his father died in 2008, allegedly calling him “the other son I would have loved to have been a mother to.” (Robinson and her husband have three grown children.) “She made sure I was OK,” McCambley told the BBC.

Soon, their relationship turned into a sexual one. When McCambley showed an interest in opening up a cafe, despite next to no business experience or funds, Iris Robinson allegedly asked two developers for loans totaling $80,000, most of which was passed along to McCambley, except for $8,000, which she is alleged to have kept for herself.

The affair ended in late 2008. In March of last year, by which time her husband had learned of her infidelity, Iris Robinson tried to commit suicide, she later revealed.

The affair has inevitably spawned gleeful comparisons to “The Graduate,” in which the middle-aged Anne Bancroft seduces young Dustin Hoffman.

“Here’s to you, Mrs. Robinson,” the Guardian newspaper splashed on its front page Saturday, invoking the Simon and Garfunkel song from the film over a photo of McCambley serving up a cup of coffee.

The people of Northern Ireland are less amused, with many responding to the Robinsons’ alleged misconduct with the stony sound of silence.

henry.chu@latimes.com