Cameras that automatically record car number plates, a weapon in the fight against crime and terrorism, could breach human rights and privacy laws, the government's surveillance watchdog warned today.

Sir Christopher Rose, the chief surveillance commissioner, said that evidence obtained by the cameras could be challenged if used in court.

Though he did not spell out his concerns in his first annual report published today, he said his position was "the same" as that of his predecessor, namely that new legislation was needed to resolve issues "arising from enhanced technological capability".

The problem is that automatic photographing of number plates, with information passed on to the Highways Agency, can be classed as covert surveillance. However, it is not covered by existing laws regulating the use of covert surveillance.

Sir Andrew Leggatt, Sir Christopher's predecessor and like him a former appeal court judge, warned last year that the deployment of automatic number plate recognition constitutes surveillance when an identifiable image is recorded of a person in a vehicle.

He added that it could also amount to obtaining private information about the person whether or not the individual had been identified in the context of a specific investigation or operation. He said the practice "will therefore be vulnerable to challenge unless it is authorised".

The trouble, the surveillance commissioners say, is that if the number plate recognition system is set up to record any vehicle which is linked to a computer database, including that of the Highways Agency's camera records, it is unlikely that the system would be authorised.

Sir Christopher made it clear that the Home Office had ignored his predecessor's warnings of the need for new legislation to protect a system widely used by the police to pursue cars suspected of being involved in crime. Whitehall officials say that the system is also valuable in tracking terrorist suspects.

The office of the surveillance commissioners was set up by the Regulation of Investigatory Powers Act 2000.