The DUP MP for Stangford, Iris Robinson, broke the law helping her 19-year-old lover start a business.
BBC Northern Ireland's Spotlight programme said she did not declare her financial interest in a deal with two property developers.
Kirk McCambley launched a cafe with £50,000 from the developers.
He claimed Mrs Robinson, who is also an MLA, then asked him for £5,000 "for herself".
She did not declare the £50,000 in the register of members' interest at Westminster or in the Northern Ireland Asssembly.
McCambley, now 21, gave an interview to the programme and revealed that "a sexual relationship began in the summer of 2008".
The Spotlight programme also reported that her husband Peter Robinson, MP for Belfast East, leader of the DUP and first minister of Northern Ireland, "became aware of the money she had received from the developers" and pressured her into returning it, but did not inform the authorities, "despite being obliged to act in the public interest by the ministerial code".
Mrs Robinson, 60, married Peter Robinson in 1970. They have three children.
In an unprecedented turn of events, she revealed yesterday that she had tried to commit suicide after an extra-marital affair.
She had announced in December her intention to step down from public life due to mental illness.
Yesterday she admitted she "encouraged friends" to assist her lover "by providing financial support for a business venture".
As well as an MP and a member of the Northern Ireland Assembly, Mrs Robinson is an longstanding and influential councillor on Castlereagh Borough Council.
The BBC reported that the council used EU funds to build the visitor centre that houses McCambley's cafe and his was the only tender for the amenity that met the criteria.
In her statement yesterday, Mrs Robinson gave an account of events.
"Over a year and a half ago, I was involved in a relationship. It began completely innocently when I gave support to someone following a family death," she said.
"I encouraged friends to assist him by providing financial support for a business venture.
"Regrettably, the relationship later developed into a brief affair. It had no emotional or last meaning but my actions have devastated my life and the lives of those around me.
"I am aware that I did not only hurt Peter, I hurt my family and friends
"Everyone is paying a heavy price for my actions. Psychiatrists may suggest that my mental illness was a significant factor explaining my irrational behaviour.
"I do not, in any way, question or doubt their judgement, but in order to master my life, I do not want to dilute the blame or resist taking full responsibility for my actions. I am completely ashamed and deeply embarrassed.
"So great was my guilt and regret that I tried to take my own life ten months ago. I have received the best of care and support both from family members and from health professionals."
Mrs Robinson also said she had let down "thousands of people who placed their faith in me and though my medical condition was a factor, I was not, at this time, true to the values, I professed. I grieve that I have damaged my profession in Christ, but I am comforted that He was able to forgive even me".
An outspoken born again Christian, in June 2008, just days after her husband took over as first minister from Ian Paisley, Iris Robinson went on BBC Radio Ulster and declared that gay people could become heterosexual through therapy and claimed homosexuality is an "abomination".
In the ensuring controversy she refused to back down and went on to link gay people to child abusers.
Peter Robinson, MP for Belfast East since 1979, said yesterday that he had considered leaving his wife after he found out about her infidelity.
"You will appreciate how devastated I have been about what you have learned from Iris," he said.
"This has been the most difficult period of my life and I have been deeply hurt by what has happened. I feel the pain of it every day.
"My first knowledge of Iris's inappropriate relationship came about midnight on March 1 last year - the night she tried to take her own life.
"Each single morning since then, I have asked for the strength to carry this burden.
"My immediate impulse was to walk away from my marriage. I felt betrayed after almost 40 years of being happily and closely bonded together.
"The circumstances I face, however, caused me to take a different course. Iris, racked by guilt and sorrow, had attempted to take her own life and would certainly have been less likely to recover if I had left.
"Over time and on calmer reflection, I set her inappropriate behaviour against 40 years of bringing up our children - often alone.
"I determined that I could not walk away without making a genuine effort to see if my marriage could be saved.
"That is the road I am on. It is a road without guarantees but not without hope."
The first minister's pained media confession and now the news that he did not report his wife for breaking the law have led to questions about whether he can carry on in office.
The political process in Northern Ireland is at a key stage, with an seemingly-elusive agreement with government partners Sinn Fein on policing and justice vital to the continuation of stable cross-community relations.
Last year Peter and Iris Robinson faced criticism after it was revealed they are paid more than £500,000 a year in salaries and expenses and employed four of their relatives at a cost of £150,000.
Mrs Robinson's comments about homosexuality have earned her the derision and emnity of the gay community and the nickname The Wicked Witch of the North.
