Usher's former wife Tameka Foster is demanding a retrial after a judge awarded the R&B star primary custody of their two young sons last month.

Foster, who divorced Usher in 2009 after two years of marriage, was stripped of custody of four-year-old Usher and three-year-old Naviyd, but she is appealing against the decision by filing a motion for a new trial.

In legal documents obtained by TMZ, Foster alleges the judge was biased in the singer's favour because of a personal link to his lawyer.

She also argues that her lawyers made a strong case against Usher's "past, current, and future inability" as a father, while there was no evidence given that she could not take care of their kids.