Air passengers travelling to British airports are to have their faces scanned and identities checked by machines under new plans.

Passengers with biometric passports are currently being scanned in a trial being carried out at Manchester Airport.

The Home Office says the machines, which compare the passenger's face to the photograph stored digitally in their biometric passport, will improve security.

Home secretary Jacqui Smith 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."

There are five new gates using the system at Manchester Airport's Terminal One, which deals with about 80,000 passengers a day. Users will also have their passports checked by a scanner to ensure that they have not been tampered with and that the passengers are not on any security lists.

Rejected passengers are redirected to immigration officers for further checks.

However, the Conservatives and unions have expressed concerns. The Public and Commercial Union, which oversees the welfare of airport staff, described the technology as "untried and untested".

A representative for the union said: "People are being allowed through on the basis of this technology. It means that 95 per cent of people won't be checked in any way, other than by the machine."

Dominic Grieve, shadow home secretary, agreed: "Instead of pressing ahead with new unproven technology to replace current immigration staff the Government should answer our calls to establish a dedicated UK border police – which is vital to securing our borders."