Standards being developed for biometric measurements to be included in all future passports could provide governments with unprecedented means of tracking people's movements, civil liberties campaigners have warned.

The International Civil Aviation Organisation (ICAO) has agreed that the initial biometric standard for passports will be facial mapping. Adequate memory space in newly issued passports will be reserved for additional biometrics.

Opponents claim this would allow governments to create a distributed international identification database for all passport holders, and that, by 2015, one billion travellers could be tracked using the information.

Privacy International and the American Civil Liberties Union are among a number of groups that have joined forces to oppose the plans.

In an open letter to the ICAO they warn: "We are increasingly concerned that the biometric travel document initiative is part and parcel of a larger surveillance infrastructure monitoring the movement of individuals globally.

"While we understand the desire of the ICAO to increase confidence in travel documents, reduce fraud, combat terrorism, and protect aviation security, the implementation of biometrics will have disproportionate effects on privacy and civil liberties."