According to Star magazine, the 27-year-old singer has gained 14lbs in just one month, to the dismay of her father and tour bosses.
It is believed that Jamie Spears had previously placed his daughter on a strict diet and exercise regime in preparation for her current Circus jaunt.
A source revealed: "[Britney] was in tears saying they're calling her fat again. She showed up to a meeting with a latte in her hand and everyone jumped on her.
"Britney's supposed to be eating a diet of fish, chicken and vegetables. When she swaps the healthy stuff for junk, her dad gets worried."
Spears has reportedly been eating fast food late at night in an attempt to fool her concerned family members.
The insider added: "Her trainers tell her they know she's cheating, but she tells them: 'Mind your own business.'"