Although the problems of facial recognition by computer are far more formidable than for car number plates, experts believe it is only a matter of time before machines can reliably pull a face out of a crowd of moving people.

If the police and security services can show that a national surveillance operation based on recording car movements can protect the public against criminals and terrorists, there will be a strong political will to do the same with street cameras designed to monitor the flow of human traffic.

A major feature of the national surveillance centre for car numbers is the ability to trawl through records of previous sightings to build up an intelligence picture of a vehicle's precise whereabouts on the road network.

However, the Home Office and police believe that the Big Brother nature of the operation can be justified on the basis of the technology's proven ability to catch criminals. "In simple terms criminals use vehicles. If you want to commit a crime, you're going to use a vehicle," said Frank Whiteley, the Chief Constable of Hertfordshire, who leads the project. " There is nothing secretive about it and we don't want it to be secret, because we want people to feel safer, to see that they are protected."

A 13-month pilot scheme between 2003 and 2004 found the performance of the police improved dramatically when they had access automatic number plate recognition (ANPR) cameras. Project Laser 2 involved 23 police forces using specially fitted vans with ANPR cameras linked to a police database. It led to a fivefold increase in the arrest rate for frontline officers.

But these mobile units will constitute only a tiny proportion of the many thousands of ANPR cameras that by next year will be feeding more than 35 million number plate "reads" every day into the new national data centre at Hendon, north London, the same site as the Police National Computer.

Mr Whiteley, chairman of the ANPR steering committee, said the intention eventually was to move from the "low thousands" of cameras to the " high thousands".

One camera can cover many motorway lanes. Just two ANPR devices, for instance, cover north and south movements through the 27 lanes of the Dartford crossing toll area on the Thames.

By March next year, most motorways, main roads, town centres and petrol station forecourts will be also covered. Some cameras may be disguised for covert operations but the majority will be ordinary CCTV traffic cameras converted to read number plates. "What we're trying to do as far as we can is to stitch together the existing camera network rather than install a huge number of new cameras," Mr Whiteley said.

More than 50 local authorities have already signed up to allow the police access to data gathered from their CCTV traffic cameras. Northampton, Bradford, Stoke and the City of London have had ANPR cameras in use for some time. Many smaller towns, such as St Albans, Stevenage and Watford are in the process of being wired up.

"We also talking to the commercial sector about their sites, particular garage forecourts. One of the biggest truisms about vehicles is that they have got to fill up with petrol," he explained.

Supermarkets are soon to agree a deal that will lead to all cars entering their garage forecourts having details of their number plates sent to Hendon. In return, the retailers will receive warning information about those drivers most likely to "bilk" - drive off without paying their bill.

The plan beyond March 2006 - when the national data centre goes live - is to expand the capacity of the system to log the time, date and whereabouts of up to 100 million number plates a day. "In crude terms we're interested in between two and three per cent of all vehicles on the roads," Mr Whiteley said.

"We can use ANPR on investigations or we can use it looking forward in a proactive, intelligence way. Things like building up the lifestyle of criminals - where they are going to be at certain times. We seek to link the criminal to the vehicle through intelligence. Vehicles moving on the roads are open to police scrutiny at any time. The Road Traffic Act gives us the right to stop vehicles at any time for any purpose. So criminals on public roads are vulnerable.

"What makes them doubly vulnerable is that most criminals not only burgle and steal, but they also don't bother to tax their vehicles, insure them and things like that," Mr Whiteley said.

Early in the new year the National ANPR Data Centre will be able to cross-check its database against all vehicles lawfully taxed and insured. All unlawful vehicles will be flagged and when they pass an ANPR camera their movements will register as "hits". The Home Office and the police believe that such a surveillance tool will have a dramatic impact on crime detection as well as the public's attitude to traffic policing.

"The first plus is that we can concentrate our resources on the vehicles we should be stopping. The other plus side is that the 97 per cent of law-abiding motorists should never be bothered by that," Mr Whiteley said.

The National ANPR Data Centre is being built alongside the Police National Computer because of the need to be constantly updated with lists of suspect drivers and vehicles. The design of the system will also take into account future changes to the way cars will be recognised, such as electronic vehicle identification - when a unique identity chip is built in to the bodywork.

Identity chips are being considered as part of a new road-pricing system based on a network of roadside radio receivers. Such electronic tags would, however, also allow a car's movements to be recorded without the need of number-plate cameras.

Asked whether ANPR will be as important as the forensic use of fingerprints and DNA profiling, Mr Whiteley replied: "It has the capability to be as revolutionary. I would describe it as an ubiquitous policing tool. You can use it in all sorts of different ways."

HOW THE INFORMATION IS GATHERED

Fixed cameras at strategic sites

Many thousands of traffic cameras on main roads, motorways, ports and petrol stations will read car numbers using Automatic Number Plate Recognition (ANPR)

Mobile units

Every force will have a fleet of specially fitted police vans with ANPR cameras. These will work alongside high-speed intercept officers

CCTV in towns & cities

Many existing traffic cameras in towns and cities are being converted to read number plates automatically as part of the new national surveillance network

CONSTANT UPDATES

Police National Computer

The PNC will supply updates on vehicles and drivers of interest to the police

Insurance data

Uninsured drivers will be identified from data provided by the insurance industry

MoT data

Vehicles without a valid MoT test certificate will be flagged

Vehicle licence data

All vehicles without a valid tax disc or with unlawful number plates will be identified

WHERE THE INFORMATION GOES

The new National ANPR Data Centre is to be based at Hendon in north London, the site of the existing Police National Computer. It is being designed to store 35 million number plate 'reads' per day, to be expanded to 100 million reads within a couple of years. The time, date and place of each vehicle sighting will be stored for at least two years, with plans to extend this period to five years. Special 'data mining' software can trawl for movements and associations

WHO USES THE INFORMATION

Police

Every police force will have direct computer access to the National ANPR Data Centre. Intelligence officers will be able to access data on a car's movements over a number of years

MI5

The Security Services have special exemption under the Data Protection Act to use ANPR information for purposes of national security. Anti-terrorism will be their main interest