A search has begun for the biological father of three sons raised by a UK-based businessman who spent two decades believing they were his own.

Richard Mason, a co-founder of Moneysupermarket.com, had his world turned upside down when a doctor told him he had been infertile since birth.

It led to the horrifying realisation that Kate, his former wife, had conceived their three children with another man during their 20-year marriage.

Mrs Mason has agreed to repay £250,000 (NZD $473,500) of their £4 million divorce settlement as part of a bitter legal battle, but the identity of her lover remains shrouded in secrecy.

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Now the devastated 55-year-old has made a public appeal for the man to come forward for the sake of his adult sons, revealing a few clues he believes may help identify him.

"They will want to know who their real father is, even if hopefully they will always think of me as their dad," Mr Mason told the Mail on Sunday.

A life-altering revelation

The life-altering revelation came in 2016, when Mr Mason was diagnosed with cystic fibrosis, a disease that is inherited and leaves men unable to father children naturally.

Distraught and confused, the businessman confronted his ex-wife, who admitted to being unfaithful over the course of several years at the end of the 1990s.

She refused to name her lover, saying only that he was a colleague at Barclays bank whom she met whenever she visited London in her role as a union official.

Previously insignificant episodes were suddenly cast in a new light, such as Mrs Mason's unexplained insistence that each son should have a Jewish middle name.

Mr Mason believes this detail could hint at the biological father's true identity.

Further DNA tests confirmed his worst fears and Mr Mason began legal action against his wife to claw back money from their divorce settlement, as well as suing her for paternity fraud.

The case was settled out of court at the end of November in an outcome believed to be the first of its kind.

His sons refuse to speak to him

But the legal battle shattered his relationship with two of his sons, who he says no longer to speak to him.

"I still see what the boys are doing on Facebook and it's heart-wrenching because we saw the graduation of the eldest on there, but I wasn't invited," he told the paper.

"And when friends post things on Facebook about their own families like their first grandchild or saying they're proud of their boy for something, I just think, 'My God, that's all been taken away from me'."

Mr Mason, who has since remarried and now lives in Rhos, North Wales, said he wants nothing more than to re-establish a relationship with his children.

He said: "If I could wave a magic wand, I would want to be in all their lives."

Mrs Mason, who lives with the children in Uttoexter, Staffordshire, was unable to be reached for comment.