The paparazzi are not known for their scruples when it comes to hounding their celebrity quarry. So the resignation of a British photographer from his LA agency in protest at the “aggressive” treatment of Britney Spears is a signal that media harassment of the unravelling celebrity has reached a new and troubling level.

Nick Stern predicted that the pursuit of Ms Spears would end in tragedy as he quit his job at Splash, a Los Angeles-based, British-owned celebrity picture agency, over the tactics employed to feed the media obsession with the troubled pop star.

"Directly or indirectly, Britney is going to come to some horrific end, or a member of the public will," he said, according to the Guardian newspaper. "It's not what's being done, it's the way it's being done. As she continues to deteriorate psychologically, I just can't see a positive way out."

In a warning that evoked disturbing echoes of the fate of Diana, Princess of Wales, Mr Stern said that the high-speed convoy of paparazzi cars and motorcycles that tailed Ms Spears 24 hours a day posed the greatest risk. Photographers drove aggressively with little regard for the safety of the celebrity or the public, he suggested, claiming that it was not unusual for cars to drive at dangerous speeds on the wrong side of the road in their efforts to snap that lucrative picture.

The feverish hunting of Britney Spears peaked in the early hours of yesterday when the paparazzi chased an ambulance and police escort transporting her to hospital for the second time in a month.

According to the Los Angeles Times, the convoy trailing the emergency vehicles taking Ms Spears to the UCLA Medical Centre stretched longer than a football field. Police and medical crews who had arrived to section the 26-year-old singer - the culmination of days of secret and meticulous planning –spoke cryptically of "delivering the package" in an attempt to fool the media. But their efforts failed as hordes of photographers descended on Ms Spears’ home, swarming her as she was stretchered out and thrusting camera lenses through ambulance windows.

"The paps are completely out of control," Mr Stern, 43, said. "It's not unusual to have 20 or 30 cars pursuing her at any one time. It's become acceptable to drive at 80mph down the wrong side of the street into oncoming traffic. I was horrified at what goes on. It's so aggressive, there are fights and crashes and slashed tyres. I felt I needed to say something."

Ms Spears was admitted to the UCLA Medical Centre on mental health grounds, reportedly on the orders of a psychiatrist who has been treating her for an alleged bipolar disorder.

The singer, once an icon of teen chastity, appears to have become locked in a spiral of self-destruction since filing for divorce from her husband, Kevin Federline, in November 2006. She has exhibited increasingly bizarre behaviour in public, including shaving her head, driving erratically and attacking a car with an umbrella.

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.

Rock bottom appeared to have been reached earlier this month when she was hospitalised at the Cedars-Sinai Medical Centre in Los Angeles for two days 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.

However after discharging herself, Ms Spears' state only deteriorated further with reports that she was using crystal meth, and, amid fears of a suicide attempt.

Her very public breakdown has fascinated the global media, with packs of journalists permanently camped outside her house, following her every time she goes out. Police were called to her 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.

The obsession with the singer's decline is such 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.

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