Only last month, Andy Burnham, the Home Office minister, said in a parliamentary written answer that there were "no plans to use radio frequency identification (RFID) tags in ID cards".

However, a leaked letter from Mr Burnham indicates that the chips will use radio frequencies to allow "contactless" reading of the card by special scanners.

The Home Office said the signals emitted would be picked up only at a distance of a few inches. But Phil Booth, co-ordinator of the No2ID campaign, said receivers could easily be boosted to receive signals from much further away. This would allow anyone carrying the card to be tracked in the street or entering a building.

Mr Booth said that unlike normal RFID technology, which simply broadcast a number as a means of identifying an individual holder, the chips envisaged for use would transmit personal details.

He added: "This technology will make the cards a snooper's paradise. It is outrageous for the Government to conceal this from the public and try to deny it in parliament."

However, he said that since there would be no legal requirement to carry the cards, the people that the police most wanted to keep tabs would not be picked up if they took the simple precaution of leaving the card at home.

Mr Burnham said the radio technology was being introduced to meet international regulations enabling identity documents to be read by scanners at airports. It was "nonsense" to suggest the frequencies could be used to monitor people's movements.

"This kind of scaremongering is designed to whip up fears about the ID cards scheme. I hope people will see it for what it is."