Plans to fingerprint passengers at Heathrow's new Terminal 5 are being probed by the data protection watchdog.
Heathrow's new Terminal 5 was opened by the Queen this month
The Information Commissioner's Office warned airport operator BAA it may be in breach of the Data Protection Act.
Under the plans, prints will be checked at the gate to try to ensure the person who checked in is the same as the person who is boarding the aircraft.
BAA said the data was encrypted straight away and destroyed within 24 hours, in line with the act.
The investigation would not delay the opening for business of the £4.3bn terminal on Thursday, the airport operator added.
The move will allow domestic and international passengers to mingle in the terminal's departure lounge.
The idea behind the fingerprinting is to make it impossible for a terrorist to arrive at Heathrow on a transit flight, then exchange boarding passes with a colleague in the departure lounge and join a domestic flight to enter the UK without being checked by immigration authorities.
But Deputy Information Commissioner 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 heard BAA's case for requesting fingerprints.
"If we find there is a breach of data protection legislation, we would hope to persuade them to put things right.
"If that is not successful we can issue an enforcement notice. If they don't comply, it is a criminal offence and they can be prosecuted."
BAA said the Border and Immigration Agency had been keen on a "reliable biometric element" when plans had been announced for common departure lounges for international and domestic flights.
Fingerprinting was selected as the most robust method by BAA, the BIA and other government departments, it said.
A BAA spokesman said: "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 said BAA was not required to involve fingerprinting in its security arrangements at Terminal 5.
"Our primary concern is that the UK border is secure and we won't allow BAA to have a common departure lounge unless they ensure the border is secure," said a spokesman.
"They presented us with this plan, which we are happy secures the border. The design of the plan is a matter for BAA."
The Queen officially opened Terminal 5, which was subject to the UK's longest planning inquiry lasting four years, earlier this month.