Beckham: Battle with Kylie Minogue in the UK charts starts Monday

"These two are particularly big," an Amazon spokesman told BBC News Online.

"The Robbie Williams book is one of our biggest celebrity author sales this year - and it's been number one on the site for a week now."

Though the retailer declined to release actual sales figures, it has announced the relative advance sales of the two pop stars.

Exclusive letter

"It's just been in the last few days that Williams has broken away to outselling Posh by more than three to one," said the spokesman.

Amazon has also published an exclusive letter from Robbie Williams on its website.

"This book is so important to me because in a way it's like a personal record of everything I've been through over the last few months," says Williams.

Williams' July weekend in Cardiff drew 120,000

"And for all you cynical readers, yes it is 100% the real thing - in fact it's about as real as it gets."

Victoria Beckham's book, at number three on the Amazon sales chart, tells the story of the singer who dreamed of stardom ever since watching the film Fame as an eight-year-old girl.

Both stars' autobiographies have been serialised in the tabloid press.