An Ohio company has embedded silicon chips in two of its employees - the first known case in which US workers have been “tagged” electronically as a way of identifying them.

CityWatcher.com, a private video surveillance company, said it was testing the technology as a way of controlling access to a room where it holds security video footage for government agencies and the police.

Embedding slivers of silicon in workers is likely to add to the controversy over RFID technology, widely seen as one of the next big growth industries.

RFID chips – inexpensive radio transmitters that give off a unique identifying signal – have been implanted in pets or attached to goods so they can be tracked in transit.

“There are very serious privacy and civil liberty issues of having people permanently numbered,” said Liz McIntyre, who campaigns against the use of identification technology.

But Sean Darks, chief executive of CityWatcher, said the glass-encased chips were like identity cards. They are planted in the upper right arm of the recipient, and “read” by a device similar to a cardreader.

“There’s nothing pulsing or sending out a signal,” said Mr Darks, who has had a chip in his own arm. “It’s not a GPS chip. My wife can’t tell where I am.”

The technology’s defenders say it is acceptable as long as it is not compulsory. But critics say any implanted device could be used to track the “wearer” without their knowledge.

VeriChip – the US company that made the devices and claims to have the only chips that have been approved by the Food and Drug Administration – said the implants were designed primarily for medical purposes.

So far around 70 people in the US have had the implants, the company said.