Britney Spears was reportedly spotted crying backstage after her much-criticised comeback performance opening the MTV Video Music Awards on Sunday.

A source told People magazine that nerves were to blame for the poor performance: “She was just plain nervous because of all the hype and [afterwards] she was embarrassed.”

“It’s been awhile since she’s done this, and she was performing in front of all her peers, on live TV. She just got out there and froze.”

“People who were at rehearsal saw something else. It was good. It really was.”

But other sources claim the problem was related to the rehearsals: while Britney arrived in Las Vegas for run-throughs on Friday, she was spotted partying both Friday and Saturday nights.

A source told the New York Daily News: “She showed up at rehearsal 3½ hours late. And she walks in with a frozen margarita! It was so disrespectful. The network took a big gamble having her open the show.”

Another source told the New York Post: “The dance number was spectacular - without her. When the stand-in was rehearsing with the dancers, in the hours they were all waiting for her, it was amazing.”

“Then Britney showed up and refused to do anything. The dancers were supposed to lift and twirl her in the air a few times, and that just wasn’t going to happen. The more complicated dance moves had to be erased because she couldn’t do them.”

Britney also vetoed the outfit picked out by the ceremony’s stylists: “MTV wanted her to wear a corset outfit. It would have looked great and covered a lot of things up, but she hated it and didn’t think it was sexy enough.”

In related news, while Britney was doing her thing in Las Vegas, ex-husband Kevin Federline threw a pool party for their two sons. The boys both celebrate their birthdays this week (Jayden turns 1 on Wednesday while Sean turns 2 on Friday) and Kevin arranged the movie Cars-themed party for around 20 guests at his San Fernando Valley home. Britney’s parents Jamie and Lynne Spears were spotted arriving at the celebration.