The news comes as it was revealed that the government is downscaling the production of cards to just 50,000 between November this year, when the project is made compulsory for nationals outside the EU, and April 2009.

The scheme, should it be rolled out nationwide, would be the biggest security printing run ever completed, with Manchester-based Security Printing and Systems (SPSL), being chosen for the initial trial.

Airside workers at UK airports will be made to carry the cards from next year as part of the trial. However, plans to introduce the scheme more extensively could be thwarted by the unions.

Last week, the TUC carried the pilots' union BALPA's motion against the scheme.

Both organisations believe the cards should be rolled out on a voluntary basis and say the target groups are being unfairly discriminated against.

A BALPA statement said: "Congress sees absolutely no value in the scheme or in improvements to security that might flow from this exercise and feels that aviation workers are being used as pawns in a politically led process which might lead to individuals being denied the right to work because they are not registered or chose not to register in the scheme.

"Congress pledges to resist this scheme with all means at its disposal, including consideration of legal action to uphold civil liberties."

A Home Office spokesperson told PrintWeek: "We're continuing to work with and are listening to the UK aviation industry, other airport employers and relevant trade unions.

"Biometric identity cards for airside workers lock identity to the individual, providing far greater assurance of identity than currently exists within the aviation sector.

"Once identity is confirmed it is easier to verify who is being employed in sensitive roles and locations at airports, bringing benefits to employers and employees and reassurance to the public.

"A fully defined identity card scheme for airside workers is still being developed and we continue to work with and listen to the UK aviation industry and other airport employers."