The police may be breaking the law by scanning car number plates, according to the Government's surveillance expert.

Sir Andrew Leggatt, the chief surveillance commissioner, said under the Regulation of Investigatory Powers Act 2000 (RIPA) it may not be legal for police to use evidence obtained from computerised video cameras.

These cameras use Automatic Number Plate Recognition (ANPR) technology. As they automatically read the number plates of all passing vehicles, the cameras should not be confused with normal speed cameras. These take photos and do not use ANPR.

Using ANPR, the police can flag up what they believe are suspect vehicles – those that may have been used or seen at a crime scene, or which are suspected of traffic violations.

According to the Metropolitan Police, across the country, 100,000 drivers have been stopped, from a total of around 32 million number plate readings. Of these, 3,000 arrests have been made by teams using ANPR.

But Sir Andrew said the police will need to get specific permission from the courts to use this evidence garnered from ANPR readings in court, because under RIPA this is 'intrusive surveillance'.

Paul Watters, head of transport policy for the AA, said: "If you get an innocent motorist who's pulled over and treated like a criminal as a result of ANPR, they might want to take issue with the law.

"But ANPR is a very effective visual deterrent, and it shows that the police are getting the people who normally get away."

The Home Office is currently reviewing the way ANPR is used. A spokesperson said: "Following the review, we will respond to the points raised by the chief surveillance commissioner."