"She would love to be slimmer when she's on stage in December," says an insider.

"She tried joining a gym in LA, but with three boys and David to look after she hasn't been once. So with advice from a friend in the fashion industry, she's got a diet plan to help her feel great for the tour, which she has been promised will work.

"The new diet is a vegan one. Victoria is cutting out all meat and processed sugar and is switching from her usual diet drinks to healthier options."

According to the source, Posh has gained weight since the Beckhams' move to the US in July this year.

"Things are not going well for them in LA," a source told Star magazine. "Victoria has been particularly upset and sobbed to David that their move may have been a mistake. She feels lonely and homesick."