The US Patent and Trademark Office (USPTO) has upheld a challenge to the US patents that cover the isolation of primate and human embryonic stem cells held by the Wisconsin Alumni Research Foundation (WARF).

The European Patent Office (EPO) has previously rejected the claims on moral grounds .

Currently, all human embryonic stem cells sold in the US are covered by the patents with suppliers having to pay WARF a license fee.

This has led to concerns in the US that the patents are too broad and discourage potentially lifesaving research or move the research to countries where the patent claims were rejected.

Patent examiners have rejected all the claims made in the patents, saying that the cells isolated by University of Wisconsin researcher Jamie Thompson appeared to be the same as, or clear variations of, cells described in the in earlier scientific papers.

WARF has two months to respond to the preliminary ruling and has said it will appeal against the ruling by every possible route including the federal courts.

"WARF has absolute confidence in the appropriateness and legitimacy of these patents. It is inconceivable to us that Thompson's discovery would be found to not be worthy of a patent," said Carl Gulbrandsen, managing director of WARF.

The patent complaint was brought by the Foundation for Taxpayer and Consumer Rights (FTCR) and the Public Patent Foundation which believe that the patents are so broadly written that they give WARF the potential to receive royalties from a broad range of stem cell applications.

"This is a great day for scientific research," said John M. Simpson, FTCR stem cell project director.

"Given the facts, this is the only conclusion the PTO could have reached. The patents should never have been issued in the first place."