According to the News Of The World, Brook became unhappy with Danny Cipriani's close friendships with other women. It is also claimed that she was tired of his mood swings.
A friend said: "Kelly and Danny are perhaps the most drop-dead gorgeous couple in showbiz. But beneath the glam exterior it has been going downhill."
Cipriani was allegedly seen soon after the split at a Hollywood party with aspiring actress Jasmine Waltz. A source at the party said: "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.
"Danny had a couple of vodkas and was happy to show his feelings for Jasmine. They were openly cuddling in front of everyone."