"We have been aware of a growing number of concerns over the performance of some of the independent certification auditors," said Rainforest Foundation director Simon Counsell.

He said its investigation called into question the total independence of the audit companies, which include SCS and Smartwood. A statement issued by the FSC in response said that the 12 global certification bodies it accredited had to undergo rigorous assessment to achieve accreditation.

The FSC has certified over 29m hectares of managed forests worldwide. Certification bodies were reimbursed regardless of whether or not a certificate was awarded or denied, it added, and rigorous quality control systems were implemented.

WWF, which operates the WWF 95+ Group, said that although it took the allegations seriously, it was satisfied that the FSC had "appropriate mechanisms in place to deal with any problems that exist within the certification system".

The 95+ Group comprises NGOs and companies such as BBC Worldwide, Beacon Press and Future Publishing, all of which are committed to using or buying paper products that come from sustainably managed forests.

Paper merchant Paperback (also a 95+ Group member) has been in the recycled market for 20 years and said it had lost count of the number of unjustified claims against certification.

Story by Andy Scott