The BBC's Mark Simpson tests out the face scanners
Facial recognition scanners are being trialled at an airport as part of government efforts to improve security and reduce passenger congestion.
The system at Manchester Airport can be used by adult biometric passport holders from the UK and Europe.
It works by scanning passengers' faces and comparing them to the photographs digitally stored on their passports.
The Public and Commercial Services Union (PCS) voiced concerns that the technology was "untried and untested".
Facial recognition technology is part of the Home Office's e-Borders programme, which is aimed at transforming the UK's border control to ensure greater security and efficiency.
Ministers believe the facial recognition technology will help identify criminals and terrorists trying to enter the UK illegally.
About 13 million people in the UK have been issued with a biometric passport, which contains a microchip holding biographical information and images.
They're going live with it before they've been able to recognise any of the difficulties there might be with the system
Hugh Lanning Public and Commercial Services Union
A further 30 million people living in the European Economic Area (which incorporates the EU countries plus Iceland, Liechtenstein and Norway) and Switzerland, have biometric passports.
There are five new gates for the system at Manchester Airport's Terminal One, which deals with about 80,000 passengers a day.
Home Secretary Jacqui Smith, who officially launched the trial at the airport, said: "The UK has one of the toughest borders in the world and we are determined to ensure it stays that way.
"Our hi-tech electronic borders system will allow us to count all foreign nationals in and out of the UK, while checking them against watch-lists."
She added: "These checks make up just one part of Britain's triple ring of security, alongside fingerprint visas for three-quarters of the world's population, and the roll-out of ID cards for foreign nationals, locking people to one identity."
Passengers at Manchester Airport with biometric passports can pass through unmanned gates and avoid queues, although they do not have to use the system.
A scanner checks their passport has not been tampered with and that they are not on any security lists.
They are then allowed through to the next gate, where a facial recognition scanner reads their face.
Rejected passengers are redirected to immigration officers for further checks.
PCS deputy general-secretary Hugh Lanning said: "This is untried, untested technology and they're going live with it before they've been able to recognise any of the difficulties there might be with the system.
"People are being allowed through on the basis of this technology. It means that 95% of people won't be checked in any way, other than by the machine."
Shadow Home Secretary Dominic Grieve said ministers were "pressing ahead with new unproven technology to replace current immigration staff".
He added: "The government should answer our calls to establish a dedicated UK border police, which is vital to securing our borders."
Liberal Democrat home affairs spokesman Tom Brake said ministers must ensure the technology was "foolproof".
"Following the Heathrow Terminal 5 embarrassment, the last thing this country needs is another major transport hub grinding to a halt," he said.
The system will be introduced at Stansted in September, and if the trials are successful it will be extended to all major UK airports.
All new UK passports have been biometric for the past two years and the Home Office estimates about seven million people will receive one every year.
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