The Information Commissioner is to investigate plans to fingerprint passengers at Heathrow's new Terminal 5.

Under the plans passengers' prints would be checked at the gate to ensure the person who checked in is the same person who boards the plane.

But airport operator BAA has been warned the security measures may be a breach of the Data Protection Act.

Spanish owned BAA says the data is encrypted immediately and then destroyed within 24 hours in line with the act.

Other airports with common departure lounge check passengers' identities using photographs rather than fingerprinting.

The Information Commissioner's Office began its investigation after a complaint by Privacy International - a civil liberties group.

The complaint states: "We believe the BAA solution is disproportionately intrusive. Even if it were to be established that passenger switching (if indeed such a problem exists) was a terrorist threat (rather than merely a breach of airline terms and conditions on transferability) then the photo option would be less invasive and would involve fewer intrusive procedures and less personal data."

Deputy Information Comissioner David Smith told the Mail on Sunday: "We want to know why Heathrow needs to fingerprint passengers at all.

"Taking photographs is less intrusive. So far we have not head BAA's case for requesting fingerpints."

He warned if the act had been breached he would try and persuade BAA to put things right - and if that was not successful they could be prosecuted.

In a statement, BAA said: "When BAA announced plans for common departure lounges, the Border and Immigration Agency (BIA) was keen on a reliable biometric element to border control.

"Fingerprinting was selected as the most robust method by BAA, the BIA and other Government departments.

"The data is encrypted immediately and is destroyed within 24 hours of use, in accordance with the Data Protection Act.

"It does not include personal details nor is it cross-referenced with any other database."

The Home Office which runs the BIA has denied telling Heathrow to fingerprint passengers.

Terminal 5, which cost £4.3billion to build, is due to open to passengers on Thursday.

The opening is not excepted to be delayed by the Information Commissioner's investigation.