Justin Bieber became intolerable when he switched to a vegan diet.

The 'Boyfriend' hitmaker's mentor Usher convinced him to cut out meat, dairy and fish but it was such a disaster, leaving Justin cranky and miserable, that his friends begged him just to eat a steak.

A source said: ''His mood took a major nose dive - he was such a brat. Soon everyone was begging him just to eat a steak. He ordered a tofu steak and tempeh tacos from Real Food Daily in Los Angeles and made a big show of spitting out the food and making gagging noises.''

Even Justin's girlfriend Selena Gomez - who initially supported his diet - urged him to quit.

The source added to STAR Magazine: ''She had encouraged the health kick but told him to drop it. She said that being vegan turned him into a jerk.''

Usher is thought to have convinced Justin to go vegan as the 33-year-old star - famed for his toned body - has been health conscious since his father died of a heart attack in 2008.