The union's assistant general secretary Tony Burke has written to the charity's chief executive David Nussbaum asking to meet with it and print industry representatives to discuss the issue.

"WWF's statements regarding the effects of printing on paper is, in our opinion, misleading in that it could lead members of the public into believing that be not using printed products or indeed printing out documents onto paper, will 'save trees'," said Burke.

"The papermaking, printing and indeed paper recycling industries provide employment for thousands of workers throughout the world, who care for the environment and who support all initiatives to help preserve and replenish forests."

The WWF said it had received the letter and that it was considering its response.

Meanwhile, Adobe, the software company that created the PDF format, has said that while it is working on developing the PDF standard, it would not support the .wwf extension.

Rick Brown, senior director, product management, Acrobat Solutions said: "Adobe Acrobat allows customers to create PDF with a range of  security permissions, including the ability to disallow printing. The WWF format is based on the PDF standard  and it is  great to see WWF leveraging PDF in creative ways. At this point, we don’t intend to support the .wwf file extension."