The 'Womanizer' singer is said to have split from DeGrazio as she has decided that she no longer wants to restrict her diet.
A source told In Touch Weekly that while the mood between the pair is still "amicable", there was tension over Spears's lack of adherence to DeGrazio's fitness expectations.
"She knows [boyfriend] Jason [Trawick] loves her no matter what and he has encouraged her not to stress about it. But Derek felt Britney made him look bad when she cheated on her diet," the insider explained.
Spears recently confessed that she often finds it difficult to become motivated to work out.
The star recently celebrated her 30th birthday.
Watch Britney Spears's video for 'Criminal' below: