Thinking about committing a crime? Already.Busted.

CCTV cameras which can 'predict' if a crime is about to take place are being introduced on Britain's streets.

The cameras can alert operators to suspicious behaviour, such as loitering and unusually slow walking. Anyone spotted could then have to explain their behaviour to a police officer.

It will also fuel fears that Britain is becoming a surveillance society. There are already 4.2 million cameras trained on the public. The technology could be used alongside many of these to allow evermore advanced scrutiny of our movements.

Civil rights campaign group Liberty is sceptical over the plan. A spokesman said: 'Bringing expensive Hollywood sci-fi to our car parks will never be as effective as having police on the street leading the fight against crime.'

The cameras, trained on public places, such as car parks, are being tested by Portsmouth City Council.

Computers analyze the movements of people or vehicles in the camera frame. If someone is seen lurking in a particular area, the computer will send out an alarm to a CCTV operator. The operator will then check the image and – if concerned – ring the police. The aim is to stop crimes before they are committed.

Councillor Jason Fazackarley of Portsmouth Council said: 'It's the 21st century equivalent of a nightwatchman, but unlike a night-watchman it never blinks, it never takes a break and it never gets bored.'

Crimes are predicted by special CCTV operators in the blockbuster Minority Report, starring Tom Cruise

Mr Hewitson added: 'Although we are a long way off Minority Report, it is a step closer.

'But what it cannot do is say whether a guy is waiting for his girlfriend or about to commit a crime. That is for the operator to make a subjective human decision on.'

The system is already successfully in use in New York and other US cities.