The sixth book in the hugely successful Harry Potter series, 'Harry Potter and the Half-Blood Prince', is already among the bestsellers on the internet - more than six months before its publication, giant US online retailer Amazon said on Wednesday.

Amazon.co.uk said advancer orders for the book, written by British author J.K. Rowling, have propelled it to number one on its list of 100 bestsellers.

"The fact that the book has already hit number one in our chart shows how incredibly excited customers are about the sixth Harry Potter," said an Amazon spokesman.

The fifth book in the series, 'Harry Potter and the Order of the Phoenix', published in June last year, received 420 000 advance orders after the announcement of its publication.

British publisher Bloomsbury said on Tuesday that Rowling, pregnant with her third child, had finished the latest and penultimate book in the series about the teenage wizard's adventures, and it will be published on July 16 next year.

The books have captivated millions of children and adults and spawned a multi-million dollar film, merchandising and publication industry.