It also introduced an opt-out for parents uncomfortable with the technology, allowing pupils to enter a four-digit pin code instead of scanning their print.

The revelation comes as teachers today warned schools are routinely taking children's fingerprints without permission from their parents.

As many as 3,500 schools take biometric data from pupils to speed up basic administration such as buying canteen lunches or borrowing library books.

Youngsters place their thumbs on a scanner and lunch money is deducted from their account or they are registered as borrowing a book.

But there were warnings today that schools are bringing in the systems without seeking parents' consent.

Teachers fear the failure to secure consent is widespread because there is no requirement in law to gain active permission, and consulting parents at all is simply seen as 'good practice'.

The Association of the Teachers and Lecturers, meeting for its annual conference in Manchester yesterday, passed a motion demanding the union investigates the issue.

Hank Roberts, an ATL executive member, said civil liberties were being eroded.

'There has been a severe diminution of civil liberties and freedoms in this country and we face the danger of more and worse to come,' he said.

'The important thing is the question of parental consent.

'Do you want your kids to have their fingerprints taken without your knowledge and without your consent? I think that's wrong. I think it's completely and fundamentally wrong.

'These things are going on now without that occurring, and it's multiplying.

'It's outrageous that children's fingerprints can be taken without parents' consent.'

Azra Haque, a teacher from Brent, said: 'Should we allow Big Brother in our schools?

'Today's children are in general much more closely monitored than previous generations.

'We really do need a strong and explicit law in this regard.'

At Capital City Academy, staff attempted last year to introduce fingerprinting technology to speed up lunch queues.

But one mother said: 'My son was frogmarched by one of the teachers to be fingerprinted even though he did not want to.

'I was just furious. There has been no consultation with parents; they just went ahead and did it.'

In a subsequent letter to parents, which sought written consent, acting principal Debbie Ramm-Harpley said: 'We would like to take this opportunity to express our sincere apologies for any confusion or distress caused as a result of the biometrics process.'

The academy's new principal, Alex Thomas, said: 'The school went through a process of letting parents know that we would be introducing the system and when it was originally conceived, consent was not actively sought.

'Although parents were made aware through a series of letters, this obviously caused some concern.'

He stressed the school had wiped the first set of scans taken before parental permission had been sought and started again with only those pupils whose parents had given their consent.

Supporters of the technology insist the data is not stored as fingerprints but unique number streams derived from the prints, rendering it useless to all but the school using the system.

But the systems have still raised concerns over the security of pupils' sensitive data.

Meanwhile Dr Emmeline Taylor, who conducted a study on surveillance of pupils funded by Salford University, has revealed that 3,500 schools in the UK - one in seven - are estimated to be using fingerprint technology.

A 2007 survey by the Liberal Democrats found that out of 285 schools using fingerprint scanners, only 48 had first sought parental consent.

A spokesman for the Department for Children, Schools and Families said: 'It is down to each school to judge best how to manage their own day-to-day running.

'But we've made clear that it is plain common sense for heads to consult parents about potentially sensitive issues like this.

'If parents want to opt out, then the school should listen to them and consider alternate arrangements.'