The model is said to have discovered racy texts to other women on the rugby star's phone over the weekend, causing her to throw him out of her house after an explosive row.
Danny confessed to The Sun newspaper: ''I can't believe what I've done.''
The 25-year-old sportsman - who rekindled his romance with Kelly, 33, earlier this year - has reportedly been sending dirty texts and pictures to women for months, even as his girlfriend nursed him back to health when he was hit by a bus in March.
A friend of Danny's told the newspaper they had ''no idea'' whether Kelly would take him back.
The insider added: ''Danny is absolutely devastated. He just can't believe what he's done - he just wants Kelly back.''
The 'Piranha 3D' actress shared pictures of the pair on a romantic holiday to Greece last week, but has now deleted all photos of the love rat from her Instagram account.
Sales manager Stacey Simcox has revealed Danny messaged her for eight months, claiming his messages got increasingly more demanding as time went on.
She said: ''At the beginning he was just friendly and really sweet. He'd ask how my day was and we would just chat away.
''But then after a couple of weeks it got really dirty. He'd send me filthy messages about what he wanted to do to me and begging to meet up for sex.
''It got progressively more explicit and I just thought, 'What a rat'... I just couldn't understand why he'd stray. Every man fancies Kelly, but that obviously wasn't enough for Danny.''
Kelly and Danny first began dating in 2008, ending things in 2010 before reconnecting earlier this year.
The busty brunette's rival, Katie Price - who briefly dated Danny in 2011 - recently said the rugby hunk would be unfaithful to Kelly and claimed he was still messaging her.
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