With blue emergency lights flashing and two helicopters hovering overhead, the troubled pop singer Britney Spears was escorted to hospital by ambulance last night in a police convoy the length of a football pitch.

Spears was admitted to the UCLA Medical Center on mental health grounds - her second hospital admission in a month - reportedly by arrangement and on the orders of a psychiatrist who has been treating her for an alleged bipolar disorder.

A police officer on duty outside Spears's mansion said that the pop star was being taken "to get help". Along with the ambulance, the pop star's motorcade included police officers on nearly a dozen motorcycles and in two cruisers.

The Los Angeles Times and the TMZ gossip website reported that a pre-prepared plan was set in motion at 1.30am local time (0930 GMT) to move Spears to hospital past the pack of journalists who have besieged her Studio City mansion in Los Angeles for weeks.

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The unconfirmed report suggested that the psychiatrist had triggered the plan after becoming alarmed at Ms Spears's erratic driving and the fact that she had not slept since Saturday. It was reported that she was placed on a 5150 hold - the US equivalent of being sectioned, the involuntary confinement of a person deemed to be a danger to themselves or others.

Her parents, boyfriend and her close friend and manager Sam Lutfi were said to be at her side.

Spears has been in an increasingly fragile mental state since she filed for divorce from her husband, Kevin Federline, in November 2006. The divorce went through in July.

She lost custody of her children, and an attempt to restart her performing career at the MTV awards in September was critically panned, although a recent album - her first in four years - was well-received.

Paparazzi photographers have snapped her in a succession of bizarre antics, including shaving off all her hair and attacking a car with an umbrella.

She appeared to have reached a low point earlier this month when she was hospitalised at the Cedars-Sinai Medical Centre in Los Angeles for two days on January 3, after a three-hour police stand-off at her mansion.

She had been refusing to hand back her infant sons Jayden James, one, and Sean Preston, two, to their father.

She was taken to hospital strapped to a stretcher and appeared to be in a distressed state. Dr Phil McGraw, the celebrity television doctor, visited her at her family's request on January 5 just before she was discharged, and later told celebrity chat shows that she was in "dire" need of medical and psychological help.

Dr McGraw later apologised for talking on television about Ms Spears' mental condition, though he denied accusations of breaching the family's trust.

Spears had until then been able to see her sons three times a week, but after the January 3 incident a court cut off her contact with them. The ruling was made on January 14 at a chaotic hearing to which the star arrived five hours late, leaving again within minutes.

Even before then, an assortment of journalists has been permanently to be found outside her house, and following her every time she goes out. Police were called to Spears's mansion earlier this week to disperse a large crowd of photographers reportedly trespassing in her grounds.

Celebrity gossip magazines and websites say that they will pay $10,000 or more for an exclusive photograph of Britney - three times as much as for an image of Angelina Jolie, the next most bankable star.

Media fascination with Spears's public mental breakdown is at such a pitch that OK! magazine has featured her on the cover of its US edition on 54 of the 103 occasions the British title has published since it launched in America in January 2006. OK! says that it retains ten people to work solely on the Britney story.

In recent days, Spears is said to have rowed with Mr Lutfi, and also with Adnan Ghalib, the British paparazzo who is her on-off boyfriend. A talk show host said on Monday that Mr Lutfi had told her that the singer suffers from "mental issues" and is seeing a psychiatrist.

Lynne Spears, the star's mother, who has had a rocky relationship with her famous daughter in the past, was reported to be staying near Britney this week and supporting her in her efforts to sort her life out.

She was seen leaving the psychiatric hospital at UCLA Medical Center at about 5:30 a.m. Asked by a throng of paparazzi and reporters whether Britney was doing all right, Lynne Spears replied, "Yeah," before leaving in a Range Rover.

Spears recently put on hold her plans to publish a warts-and-all account of raising her children after her younger daughter, Jamie Lynn, who is 16, revealed that she is pregnant.

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