At last week's Frankfurt book fair, the environmental organisation highlighted a study that questioned the ethics behind the production of German children's books, claiming 19 of 51 books tested showed "significant traces of tropical wood".

The books came from well-known publishers.

Nina Griesshammer, a WWF forest expert, said: "The types of tropical wood found in the books such as Shorea or Rhizophora do not typically occur in plantations, but rather almost exclusively in natural and virgin tropical forests.

"The logical conclusion is that for those books which tested positive for tropical wood, natural tropical forest was destroyed."

The body suggested that, as more than a third of books imported to Germany originated in China and nearly half of Indonesia's pulp exports are sent there, companies that produce pulp there, naming APP as one, are responsible for destroying the tropical forests in this way.

A spokesperson for APP said: "We are surprised to be mentioned in this context. Of course, no one involved would appear to be able to state where these publications were printed, or on what quality from which mill, so the comments made become quite general."

The company said its raw material is 30% certified, which it claims is "significantly" above global standards. It said it maintains a strict Fibre Procurement Policy, which states that the company's forest-to-mill gate protocols aim to "ensure no illegally obtained wood enters the fibre supply through a strict, independently audited, Legal Origin Verification and Chain-of-Custody".

It also claims to have zero tolerance for illegal wood and that it demands its pulpwood suppliers ensure the legality of the wood, if possible, via certification.

Read associate editor Jo Francis's blog: WWF Worry