ANPR devices work by scanning vehicle number plates and identifying registration numbers.

The police use ANPR to check registration numbers against information stored in databases, including the Police National Computer, to identify vehicles of interest, such as stolen cars or those involved in crimes. When a suspicious vehicle is recognised it can be the focus of targeted interception and enquiries.

The vehicle registration number, date, time and location of the ANPR 'read' are recorded. In addition, an image of the number plate and a wider shot of the vehicle are usually stored to allow manual verification that the registration number was read correctly.

All equipment connected to the police service National ANPR Infrastructure has to conform to strict National ACPO ANPR Standards that set out detailed technical specifications. However, optical character recognition is affected by factors such as angle and lighting, so images are stored to allow accuracy to be checked prior to any action being taken.

The National ANPR Infrastructure reads approximately 10-15 million number plates per day and the National ANPR Data Centre has around 12 billion records in it (as at May 2011).

It has been agreed with the Information Commissioner's Office that the police can retain ANPR data for up to two years, after which time all records will be deleted unless they are being used in a live investigation.

Police ANPR data is collected and used solely for policing purposes, and it is not sold or provided to any non-law-enforcement agencies or companies.

There are strict guidelines that limit the purposes for which the local and national police ANPR databases may be interrogated, and all actions taken are logged and are able to be audited.

For operational reasons, we are unable to disclose the specific locations of fixed-site police ANPR cameras. However, you should expect ANPR cameras to be in use on many roads across the country and in locations requiring enhanced security.

Police forces are encouraged to display appropriate signs on vehicles that are equipped with ANPR cameras.