The 31-year-old bricklayer scooped the top prize on the first episode of Simon Cowell's ITV1 gameshow on Saturday night. However, it emerged yesterday that Hageman spent two and a half years for actual bodily harm and aggravated burglary.
Speaking about his conviction, he told one paper that he had been an "idiot" for breaking into his male victim's home and attacking him.
© ITV
However, both The Sun and The Mirror today claim that Hageman's victim was a woman - his ex-girlfriend Amy Edwards.
"The victim of the assault occasioning actual bodily harm named in the indictment was Amy Edwards," a court spokesperson confirmed.
Edwards declined to comment, but a source reportedly told The Sun: "It was a vicious beating. Hageman claimed he had attacked a man, but in fact it was Amy who he assaulted. She was left with some very nasty injuries. He is not some brickie with a bit of a violent past. He is a woman-beater.
"Amy dumped him and he beat her up. She was petrified to be in her flat on her own because he threatened to kill her via text messages, emails and phone calls."
The insider continued: "One night she got a friend to stay. It was about 1am. She was in bed and he climbed in the window. He'd stolen a ladder from a neighbour. He punched and kicked her - she was a mess."
Meanwhile, the sister of Hageman's current girlfriend Jane Makepeace - seen celebrating his win on the ITV1 gameshow - claimed that he had also been violent towards the 29-year-old.
"They've had a real up-and-down relationship over the last two years, very hostile. Jane was in hospital at one stage with a suspected bleed on the brain," Rebecca told the paper. "I think it was a punch in the back of the head. She couldn't open her eyes. She couldn't move.
"My mum called the police. She couldn't understand why nothing ever came about. Police were involved quite a lot to be honest."
© ITV
Domestic violence campaigner Dr Linda Papadopoulos, MP Hazel Blears and former MP Ann Widdecombe are among those who have called on the winner to be stripped of his £1m prize.
"It seems wrong that someone like him should even be allowed on game shows like this, and it is all the more sickening that he has won a million pounds," Blears said. "Domestic violence is one of the most horrible crimes, committed against vulnerable women and sometimes men. In light of his conviction, he should be stripped of his winnings and the money given to a charity supporting its victims."
Simon Cowell's spokesman Max Clifford said: "Nathan's past has come as a complete shock to Simon. He'd no idea about the attack on his ex. He's holding urgent meetings today to get to the bottom of the matter and come up with a response."
Broadcaster ITV said: "In light of developments we are making enquiries to establish the facts."
Red or Black? continues tomorrow at 8pm and 9.30pm on ITV1.
> 'Red or Black?' Nathan Hageman win branded "sickening" after criminal past emerges
> Simon Cowell's latest 'Red or Black?' contestant Ian Murphy loses out on £1 million
