Pop star JESSICA SIMPSON's romance with TONY ROMO collapsed when he found text messages from her ex-lover JOHN MAYER on her cell phone, according to new reports.

The couple is alleged to have split last week when the Dallas Cowboys quarterback ended the 20-month relationship on the eve of Simpson's 29th birthday on Friday.  The stars have yet to confirm the break-up claims - but reports suggest Romo dumped the singer when he discovered she was still in touch with rocker Mayer, from whom she split in 2007.

A source spills: "They (Simpson and Romo) were hanging out and he picked up her phone and she got defensive about it. Tony found messages from John and went ballistic. He dumped her right then. He walked out and that was it."