TMZ.com reports the 26-year-old singer had to be wheeled out of her Beverly Hills mansion by paramedics and taken to the Cedars-Sinai Medical Center on a “medical hold” after police found her under the influence of an “unknown substance”.

Pagesix.com reports that sh has lost a tremendous amount of weight due to “mental stress and anguish” over her ongoing custody battle for her two children with ex-husband Kevin Federline.

Officers were called to the singer’s home about 8pm (local time) after she reportedly refused to return custody of her two sons Sean Preston, 2, and Jayden James, 1, back over to their father’s bodyguard as part of court orders.

Los Angeles Police Department spokesman Jason Lee said officers had been called to the home to deal with a “family custody dispute”.

“While officers were on the scene, they observed Ms Spears under the influence of an unknown substance,” Lee said.

“She is now being taken in for an evaluation to a hospital.

“There were no reported injuries to anyone involved including the children.”

Lee would not confirm who made the call requesting police assistance.

Spears, who was nearly two hours late for yesterday’s scheduled hearing in her custody case, was reportedly scheduled to have her sons until 7pm last night, when she was supposed to hand over the children to one of Federline’s employees.

A bodyguard for Federline, 30, arrived to pick up the boys at the designated time, but Spears reportedly refused to relinquish her sons.

Police said officers determined that the court order stated that the singer’s children were supposed to be with their father and a transfer was arranged about 11pm.

Federline’s lawyer Mark Vincent Kaplan was at Spears’ home about 8pm dealing with police on behalf of his client but left the home before the transfer was made.

In addition to multiple officers, fire trucks, two ambulance and a police helicopter were all on the scene as the drama unfolded.

Spears, who was sporting long brown hair extensions, was taken to an ambulance on a gurney and driven to Cedar-Sinai Medical Centre.

TMZ reports that Spears was smoking inside the ambulance and made rude gestures at a group of waiting paparazzi as she was wheeled out at the hospital.

Spears’ lawyer Sorrell Trope, who this week announced she planned to part ways with the troubled singer, said police had followed the law in attending the scene and facilitating the return of the children to their father.

“In a normal case when someone shows up with a certified court order saying the kids need to be somewhere else, the police see to it that the order is obeyed,” she said.

The incident is the latest in a turbulent past 15 months for Spears, who married Federline, a former back-up dancer, in October 2004.

The Toxic singer filed for divorce in November 2006, with the split becoming official in July.

The pair have since battled for custody of their sons, with Spears’ custody strictly regulated by the court.

Since Spears became single, her troubles have played out in the tabloids.

She was photographed at several Hollywood hotspots partying with Paris Hilton, with some photos infamously capturing Spears without panties.

The former squeaky-clean teen queen then shaved her head, was filmed beating a photographer’s car with an umbrella and was then admitted to rehab for a month-long stay.

In September, her MTV Video Music Awards performance, which was meant to launch her comeback, was universally panned.

The following week, Spears’ management firm dropped her and her divorce lawyer resigned.

She then was charged over a hit-and-run incident and driving without a valid license for allegedly crashing into a parked car in August.

In October, a judge ruled that Spears engaged in “habitual, frequent and continuous use of controlled substances and alcohol”.

He ordered the pop star to undergo random drug and alcohol testing twice a week as part of her ongoing custody dispute with Federline.