Authorities will store details of car journeys surveilled by the new national Automatic Number Plate Recognition (ANPR) system for five years, the Home Office has revealed.

Senior police officer had said the data on millions of vehicles would only be kept for two years, the Guardian reports. The Information Commissioner's Office (ICO) is examining a complaint by Privacy International over the extended retention period. The privacy advocacy group described it as "unnecessary and disproportionate".

The ICO said: "Prolonged retention would need to be clearly justified based on continuing value not on the mere chance it may come in useful."

The ANPR system, headquartered in Hendon, north London, will be fully operational in the new year. Half of English and Welsh police are already supplying it with data from their upgraded CCTV networks.

Forces are being encouraged to "fully and strategically exploit" its potential for tracking suspects and vehicles by the Association of Chief Police Officers. It's envisaged that ANPR will be used as part of "mainstream policing", ranging from clamping down on uninsured drivers to counter-terror operations.

Campaigners have called for the government to more clearly define which other agencies will be allowed to interrogate the database and for what reasons. ®