Kelly Brook dumped fiancé David McIntosh because he stayed too close to his ex-girlfriend.
David sent texts and emails to his ex who he broke up with just weeks before he started dating the TV model.
The personal trainer, 28, even visited her home while Kelly, 34, was working 3,500 miles away in America.
But David, who played Tornado on TV show Gladiators, claimed he stayed in touch with his former lover because they jointly owned a dog.
Representatives for both Kelly and David refused to comment on claims the model angrily confronted the ex in a series of online messages.
But speaking to another Sunday paper, the ex claimed Kelly knew of their ongoing relationship and sent texts telling her to stay away from the bodybuilder.
She said: "This has gone on for months and I have never said a word, David originally cheated on me with Kelly and I kept my mouth shut.
"David begs and begs to see me. I've done Kelly a favour because she simply does not know the man she was about to marry."
Today's claims come after Kelly announced she was splitting from David on Friday, six months after she revealed they were engaged.
She broke the news on Twitter and posted on the site again yesterday to voice her heartbreak with a cryptic message.
Kelly, who briefly split with David earlier in the year, wrote: "I will always do anything and everything to protect the people I love even at the cost of letting myself down at times."
She also left the message: "Positive changes - good people - happy life - creative work."
Just three days ago Kelly was photographed in Los Angeles proudly wearing her engagement ring.
But a source close to her said she was shocked to hear a number of allegations about David the next day.
Kelly was spotted without her ring shortly after she announced the break-up, while former Celebrity Big Brother contestant David was said to be "heartbroken" at home in the UK.
The star also went on social networking site Instagram and removed dozens of pictures of herself with David, which were uploaded during their nine-month relationship.
Quoting American novelist John Irving on the site, she wrote: "Everyone has a right to be a little happy, a**hole."
It is believed David dated his ex for about a year before they parted company and he began seeing Kelly.
David, who met Kelly at her London restaurant in January, is understood to have insisted that the scores of messages he sent to his ex were innocent.
One evening in May - while Kelly was working in the US - David travelled 250 miles from his home in Manchester to visit his ex in Surrey.
On that day they texted each other as he travelled south by train and she made arrangements to collect him from the railway station close to her home.
It is not known when David left her house. But it is understood he has claimed that Kelly - who has publicly described him as "the most wonderful man I've ever met" - knew he was making the trip and did not mind. At the time, Kelly was working on a hectic schedule of acting and modelling projects.
Earlier this month she and David were pictured holding hands and smiling during a sunshine break on the Greek island of Mykonos.
Before that, they were photographed frolicking happily on a beach in Miami with her engagement ring on show.
They split briefly in February but got back together in March before Kelly announced they were to marry.
She posted a photograph of them on Instagram holding a balloon that read "Engaged".
In an interview shortly after the engagement she told how David popped the question at his flat in Manchester. She said: "I saw this beautiful diamond ring and I think my first reaction was, 'Are you sure?' He assured me he was, so I said, 'Of course I will.' "He slipped the ring on my finger and we kissed and it was just the most humbling, beautiful moment of my life. It was so genuine."
David, who has a two-year-old son with former Miss England Rachel Christie, said meeting Kelly had changed his opinion on marriage.
He told an interviewer: "I had the view that it was outdated and unnecessary. But meeting Kelly changed all that." In her tweet announcing the break-up, Kelly said: "It's a sad day but I wanted to share with you that David and I are no longer engaged. I love and respect him and wish him all the best."
Kelly, who was engaged once before to film star Billy Zane, has recently published raunchy memoirs in which she included intimate details about him and ex-boyfriends Jason Statham and Thom Evans.
She has also dated England rugby player Danny Cipriani.
Kelly was once asked if she thought there was a similarity in the men she was attracted to, she replied: "Yes, they've all been on their a***s when I met them. They didn't have a career or they were at the end of one career and trying to start something else.
"I met them all at not a very good time for them and that's my fault for being nurturing."
When asked about the latest split, her agent said: "We have no comment to make."
A spokesman for David also refused to comment.
