RFID chips, which are currently being implanted in humans in some US states, have been shown to cause cancer in laboratory animals, according to reports in Associated Press.

A series of studies claims to show that laboratory animals suffered cancer rates as high as 10 per cent per implant in tests conducted in the 1990s. The animals were found with malignant tumours encasing the chips themselves, in some cases.

"The transponders were the cause of the tumours," said Keith Johnson, a retired toxicologic pathologist, in a phone interview to explain the findings of a 1996 study he led at Dow Chemical in Midland, Michigan.

RFID chips have been approved for many years for use in pets to help identify lost animals, and vets have found no incidence of long-term health problems.

However, leading oncologists have stated that large scale studies should be carried out to see whether there is a problem for humans.

"Over the past 15 years, millions of dogs and cats have safely received an implantable microchip with limited or no reports of adverse health reactions from this life-saving product, which was recently endorsed by the US Department of Agriculture," said VeriChip, one of the leading providers of RFID chips for use in humans.

"These chips are a well-accepted and well-respected means of global identification for pets in the veterinary community.

"Veterinarians would not continue to prescribe pet microchips if they believed they presented significant risk of malignant tumours in dogs and cats. "

VeriChip's RFID chips have been implanted in over 2,000 human patients and are used to make sure that doctors get the right treatment to those who are suffering when medical records are not immediately to hand.

The chips are also used to allow companies to monitor staff location and even to allow entrance to some nightclubs.

But the Associated Press article also raised questions about how RFID chips came to be used in humans.

It claims that the chips were approved by the US Food and Drug Administration (FDA) but that no mention was made of any studies linking the chips to cancer.

The article also noted that the FDA is overseen by the US Department of Health and Human Services (DHHS) which, at the time of VeriChip's approval, was headed by Tommy Thompson.

Two weeks after the RFID approval took effect on 10 January 2005, Thompson left his Cabinet post, and within five months was a board member of VeriChip Corporation and was compensated in cash and stock options.

"I did not even know VeriChip before I stepped down from the DHHS," he told AP in a telephone interview.

However, it may be that the cancer studies were a result of testing procedure rather than a problem with the chips themselves.

Dr Cheryl London, a veterinarian oncologist at Ohio State University, said: "It is much easier to cause cancer in mice than it is in people.

"So it may be that what you are seeing in mice represents an exaggerated phenomenon of what may occur in people."