"I'm just winding the tape forward five years, to when we all say, 'Oh, my God!"' said Richard Smith, a US security expert who was among the first to question the plan.

And Marc Rotenberg of the Washington-based Electronic Privacy Information Centre said: "There's no doubt there are serious privacy concerns."

The new proposal is being made by the Internet Engineering Task Force (IETF), an international standards body. It would result in the inclusion of the unique serial number for each computer as part of its expanded new Internet protocol address.

These "IP" addresses, planted within e-mails and all other information flowing across the Internet, must be as unique as telephone numbers to distinguish each computer on the global network and to guide the billions of bits and bytes flowing among them.

The IETF's top engineers accept their may be implications for online privacy, but Fred Baker, the task force's chairman, said: "I think the privacy concerns are overrated."

Name and a number

The new address scheme, called "IPv6," would not become widely used for years but ultimately would affect every Internet user.

Critics warn that commercial Internet sites, which already routinely record IP addresses, could begin to correlate these embedded serial numbers against a someone's name, address and other personal details, from clothing size to political preference.

The task force itself will ultimately decide whether to include the identifying numbers in the new IP addresses. The timing on that decision is unclear.

However, Mr Baker said the task force is also looking at ways to configure Internet devices manually so that addresses will not contain the sensitive numbers.

"Those folks concerned about the privacy issue could use this (alternate) technique," said Thomas Narten, an IBM software engineer working with the IETF.