Cipriani has moved out of the 29-year-old's Notting Hill apartment and returned to his mum's home, The People reports.
Friends of the model and TV presenter said that she hasn't seen the Wasps rugby player since she returned from Los Angeles ten days ago.
A friend said: "We fear things have hit the rocks and their romance is over. If so, it's very sad as Kelly was smitten with Danny and was always smiling when she was with him."
The couple began dating in September following Brook's split from Billy Zane.