The end of the pap show

The Hollywood star Scarlett Johansson was heading for Disneyland to enjoy a day off with friends when a swarm of photographers began chasing her Mercedes. A tense game of cat-and-mouse ensued, lasting 45 minutes and only ending when Johansson's car crashed into another that was carrying a family.

Nobody was hurt, but the actress was so shaken by relentless hounding from the paparazzi that she fled her home in Los Angeles for a while last year. From today, however, such aggressive tactics will be banned when a law comes into effect in California, designed to curb a new breed of baying photographers.

Dubbed the "stalkerazzi", they stop at nothing to get the right shot. Lying in wait for a snatched shot is not enough. Instead, some engineer the action by goading, confronting or pursuing their prey. Many reckon the damage caused by ramming a lens into someone's face or crashing their car will be worth it, because the returns on their shots are so lucrative. With worldwide syndication, pictures can be worth hundreds of thousands of dollars.

But the new bill, signed into law by California Governor and actor Arnold Schwarzenegger, will hit the photographers in the pocket. They will be forced to pay three times the damage they cause and will be unable to profit from any shots taken. Cindy Montanez, the US legislator who drafted the bill, said: "Now the paparazzi are going to have to think twice about chasing a celebrity anywhere in California."

British stars will be keeping a keen eye on how the law works. Some of them - including the singers Liam Gallagher and Chris Martin and the actor Jude Law - have been involved in tussles with photographers on the streets of London, where provocative behaviour does happen. Others who have famously had run-ins with unwelcome snappers include Michael Douglas and his wife Catherine Zeta-Jones.

"This is about paparazzi who wait and hunt the celebrities, their prey, until they catch the celebrity in a state of compromise," said Ms Montanez.

But the LA-based British celebrity photographer Frank Griffin said assaults were already covered by existing laws and the new legislation would unfairly target celebrity photographers. "Why should there be different standards for a hard-news photographer and a celebrity photographer?" he said. "The law only applies to photographers who commit an assault in the course of obtaining their images. Basically what I would have to do to break the law is get Sylvester Stallone, for example, cornered, put my cameras down, give him a smack, take a picture of him going "Ouch, my jaw!", sell that picture for thousands of dollars - and then the penalty would be three times the thousands of dollars I'd get for that picture. But this doesn't happen. This law is La-La land."

Ms Montanez's law was prompted by a number of incidents in the US last year. Alongside Johansson's experience, Mean Girls actress Lindsay Lohan was involved in a crash with a photographer who was later charged with assault with a deadly weapon: a van. Legally Blonde star Reese Witherspoon was chased from her gym by paparazzi who then allegedly blocked her path. She had to seek help from a security guard.

And always in the background must be the spectre of the most infamous pursuit, when Diana, Princess of Wales was chased at speed through Paris on the night she died in 1997.

Lord Soley, who has campaigned on media privacy issues, felt Diana's death had served to curb the worst excesses of the paparazzi in the UK. "Things are by no means as bad as they once were. Because of the way she died the press pulled back a bit. I don't think something like [this law] is needed over here at the moment, but watch this space."

Problems with paparazzi have prompted some stars to adopt elaborate rituals to avoid their unwanted gaze. Robbie Williams spent a period wearing identical clothes each day when he left his Notting Hill home and wearing a mask of his own face. He figured that if photographers had pictures of him looking the same every day, they would be unable to sell them.

Additional reporting by Robin Stummer