The showbiz couple - who have been dating for nearly two years - are believed to have parted ways following a period of tension about her moving to Australia so Danny can continue his rugby career.
A source told the News of the World newspaper: "Kelly and Danny are perhaps the most drop-dead gorgeous couple in showbiz. But beneath the glam exterior it has been going downhill."
Danny, 22, has even been seen kissing actress Jasmine Waltz while at a glitzy party in Los Angeles earlier this month. An insider at the party said: "Danny seemed to be having a great time swanning around the party with Jasmine.
"She was introducing him to people, but they appeared more caught up in each other. At one stage they got very close outside by the pool and he kissed her. It was very odd bearing in mind all the English people at the party thought he was still dating Kelly."
Jasmine even revealed she had been getting close to the rugby star last week, saying: "Danny is very special and very cute. We had a great time together on Saturday (12.06.10). It was a brilliant party."
Kelly recently revealed her plans to move to Australia, saying she was bored of the UK. She said: "Although I love the UK, you kind of hit a glass ceiling a little bit here. And I get bored if I stay in one place for too long.
"My mum is in a good place now, my family is cool, my boyfriend is ready to move... the timing of everything is great."