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Under the usual procedure, evidence collected from a scene - anything from cardboard boxes to plastic bags - is taken to a laboratory and tested for fingerprints. However, because of the chemicals used, evidence is often destroyed in the process, leaving only photographs to prove the evidence existed. However, a growing number of suspects are claiming these have been altered between the time of collection and the courtroom.

In an effort to break down these types of defences, the Wiltshire Constabulary

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is pioneering a revolutionary digital storage and watermarking technique, already in place in the US. 'What we're using is a digital imaging workstation that watermarks fingerprints. We had to have the ability to prove in court that the documents had not been tampered with,' says Esther Neate, senior fingerprinting development officer at the constabulary.

The system photos of the evidence are taken using a digital camera, saved as a 13Mb file and implanted with a unique digital watermark. As well as the increased authenticity, the system has speeded up the process. 'This means that we can process results far, far quicker than we used to - in the past it took two or three days, now it can be done in a matter of minutes. If in six months you need to present images to the court you can download them straight from the CD,' says Neate.

The watermarking software is supplied by Signum Technologies of Witney, Oxfordshire. At present, Wiltshire constabulary is the only UK police force to adopt the system, but Neate says she sees no reason why other forces shouldn't follow suit.