Cooper career behind Renée Zellweger split?

Bradley Cooper's increasingly busy acting career is said to be the reason behind his split from girlfriend Renée Zellweger.

The pair, who began dating in August 2009, were rumoured to have ended their relationship earlier this week, though reps for the stars have yet to comment on the reports.

"If I had to pick any possible mistress it would be Brad's career. He worked really hard to get into leading man status," a source told US Weekly.

"Renée had to take a backseat. She travelled with him to make it work."

The insider went on to claim that Zellweger was more dedicated to the relationship, adding: "She's accomplished a lot of her professional goals. So Renée took some time out to be a great girlfriend and see if this is what it took to make a relationship work.

"She stopped fighting. It was just not working out."

It was recently reported that Zellweger visited Sandra Bullock in New York City to seek solace from the break-up.