Britney Spears' bid to win back custody of her children has been dealt a devastating blow after Kevin Federline's lawyer called an emergency court hearing for Friday (04Jan08). The troubled singer held sons Sean Preston, two, and Jayden James, one, captive in her Beverly Hills home for four hours following a scheduled visit on Thursday (03Jan08), sparking a dramatic stand-off with emergency services. Spears was eventually stretchered into an ambulance and driven to Cedars-Sinai Medical Center in Los Angeles for a psychiatric evaluation. The star's disregard for a court order that stated she must hand the children back to Federline's bodyguard at 7pm local time and the ensuing stand-off could spell the end of her custody battle with Federline, according to celebrity blogger Perez Hilton. A source says, "Mark Vincent Kaplan (Kevin Federline's lawyer) will likely ask for an ex-parte hearing first thing Friday morning." Police helicopters, cop cars, ambulances and fire trucks descended on Spears' home, with additional back-up vehicles and officers appearing throughout the ordeal. The incident is being referred to as a "family custody dispute" by the Los Angeles police and reports suggest the singer was "under the influence of an unknown substance". Spears lost physical custody of her children in October last year (07), just months after her divorce with Federline was finalised.