The perpetually pouty singer - dressed in her trademark dark sunglasses - was snapped by paparazzi buying the diet guide in an LA boutique last month. Within hours of the photo appearing on the web, the book had jumped from 77,939th place on the Amazon website sales chart to 209th - a whopping increase of 37,000%.

The sassy book - described as a "no-nonsense, tough-love guide for savvy girls who want to stop eating crap and start looking fabulous" - had failed to garner much attention prior to being lifted off the shelf by the rail-thin Posh.

Now its authors are reaping in the cash as thousands of readers turn to the unconventional book for quick weight loss advice.

The book was written by two LA fashion luminaries - former model Kim Barnouin, who has a degree in holistic nutrition, and ex-Ford model agent Rory Freedman. "They may be bitches," the book warns. "But they are skinny bitches."

Not for the faint hearted, the in-your-face book is loaded with strong language and no-holds-barred advice such as, "you are a total moron if you think the Atkins Diet will make you thin", "soda is liquid Satan", and "coffee is for pussies".

Vegan

Based on a vegan philosophy, the Skinny Bitch guide encourages women to eat whole grains, fruits and vegetables while urging them to abandon dairy products, eggs, meat and fish. The authors also issue a scathing attack on meat eaters, calling those who choose to eat meat while attempting to lose weight "morons".

"As with any diet, if you follow it to the letter then you will lose weight, but for your average woman it's not particularly easy to follow and you would have to be incredibly dedicated," said TV nutritionist and diet expert Amanda Ursell.

In the United States, veganism came under recent close public scrutiny following the death of a six week-old baby who was fed only apple juice and soy milk by his vegan parents. Last month the couple were sentenced to life imprisonment on murder charges.

Exactly what the already excessively thin Posh - who has long battled rumours of anorexia - expects to glean from the Skinny Bitch book has once again lifted the lid on the debate about celebrity weight loss.

Posh, who according to one newspaper has a 23in waist size - the equivalent of a seven year-old girl - has admitted in the past to suffering from an eating disorder. The 33-year-old mother of three - who wears a UK size 4 and US size 0 - said that she first began to lose weight at the height of the Spice Girls phenomenon in the mid-1990s.

"I became obsessed with what I looked like," she revealed in the April edition of Heat magazine. "I would look in the mirror and check the size of my bottom, see if my double chin was getting smaller.

"I began living on vegetables and nothing else, but it never occurred to me I had an eating disorder, because I was the same size I had always been. I thought I was just getting into shape."

A diagnosis of polycystic ovaries caused her to change her eating habits, but the star admitted that by then the 'damage was done', and that it was hard to resume eating normal food.

Despite published reports of her bizarre eating habits - chewing food excessively, declining portions that can't fit into the palm of her hand (the same size as her stomach), only eating fruit until 3pm and then limiting her caloric intake to 500 for the remainder of the day - Posh claims she no longer has an eating disorder.

"I do control my eating. But I think there is a big difference between someone having an eating disorder and someone who is controlled about what they eat," she told Heat. She admits, though, that she is anxious about her looks. "I know I'm not naturally beautiful or a supermodel," she said. "I'm very insecure about the way I look."

Defence

Her husband, David Beckham - who will join the LA Galaxy team on July 1 - is steadfast in his wife's defence. "I see Victoria every day of course and I make sure she eats well and she eats healthy. She looks after the way she is, you know, it is down to that person," he told one newspaper. " I love her whatever she looks like, whatever weight she is," he said.

David Beckham may be open minded about body types, but young women today are bombarded with images of scrawny celebrities like Beckham, Nicole Ritchie and Kiera Knightley.

In a recent report, the British Medical Association called this hyper-thin look "both unachievable and biologically inappropriate". Still, such common sense warnings tend to be overwhelmed by the onslaught of the celebrity media culture, especially when the same celebrities are seen reading books like Skinny Bitch.

Perhaps medical experts could persuade Posh to be photographed reading a serious report on the dangers of extreme diets. Until then, Skinny Bitch will likely stay at the top of the bestseller lists.