Electronic cards designed to store information about students' attendance are increasingly being used by pubs and shops to check the holder's age when buying alcohol or cigarettes.
The shift in the use of Connexions cards, which are available free to all 16 to 19-year-olds, has alarmed civil rights campaigners.
Connexions, the pastoral and careers guidance service set up by the Government, introduced the cards so pupils could collect points for attendance which they could then redeem for prizes and shop discounts.
More than 500 schools and colleges regularly update points on their students' cards, most by emailing Connexions and some by using electronic scanners.
But private contractor Capita, which co-ordinates the card, has also been selling special card-readers to local authorities for use in bars and shops. The readers cost around pound;18.
Luton council has bought 500 of the devices for town-centre shops using funding from the Office of the Deputy Prime Minister.
A council spokeswoman said traders could accept other forms of identification from teenagers, such as passports or drivers' licences, but the authority wanted them to use Connexions cards as the only means of ID once the cards had been provided for all teenagers in the borough.
"They're very easy to use," she said.
"You can tell whether the card is valid and it shows you the young person's age rather than leaving you to figure out from their birth date whether they are 17 or 18."
The card-readers have played a central role in Luton's Agecheck campaign, which has seen the authority prosecute 10 traders in three months for selling cigarettes or alcohol to underage shoppers.
Other councils to buy the readers include Kent and Hull, which used them in a clampdown on underage drinking at Christmas.
Liberty said it recognised some form of identification was needed to stop underage young people buying cigarettes and alcohol, but would be concerned if they were being restricted to one identity card.
"Connexions cards are becoming an identity card by stealth," a spokesman said. "It's totally against the trust and respect that the Government says the card has for teenagers."
Connexions cards are due to be replaced by "opportunity cards" under plans in the Government's youth green paper.
These cards will contain even more data, with parents and local authorities able to credit them with money for young people to spend on sports and cultural activities.
A DfES spokesman said that the Connexions cards were strictly voluntary and it did not want teenagers to be placed under an obligation to carry them.