Publishing rights on potion recipes and other book-based merchandising spin-offs from the Harry Potter series have been bought by author JK Rowling's publishers, Bloomsbury.

The publishing house, which is already thought to make 20% to 30% of its revenue from Pottermania, will produce notebooks, diaries, calenders, journals, address books and writing paper to cater for the demand anticipated from next year's big budget film version of Harry Potter and The Philosopher's Stone.

The rights have been bought from Warner Bros - who are producing the film - for what is thought to be about £100,000.

"The merchandising will be launched in March 2001 and print run quantities will be restricted to initial demand," the publishing house said.

"But early piloting of the merchandise in the US indicates that there is a very strong market for the products."

Guy Bell, an analyst at Beeson Gregory, said Bloomsbury were wise not to anticipate too much from merchandising sales.

"It is taking a cautious view of how many they are going to print," he said. "They are more than aware of the Dorling Kindersley scenario."

British publisher Dorling Kindersley ran into trouble last year after hugely over-estimating the demand for merchandising spin-offs from the film Star Wars: Episode One.