Israeli security experts have developed a way of digitally encrypting passport pictures or signatures so that they can be viewed only by customs officers.

Developed by a team at Ben Gurion University, the system works by converting fingerprints or signatures into numbers that slightly alter the position of the millions of different coloured dots that make up a normal colour photo.

According to New Scientist magazine, the documents can be faxed, scanned and reprinted without the hidden data disappearing.

To reveal the identification, the picture has to be scanned and then run through a special computer program to reconstruct the fingerprint or signature.

Joseph Rosen and his team of scientists are now designing a handheld reader to unscramble the information.

The technology is a variation of a system developed by paedophile ring The Shadowz Brotherhood which used apparently innocent picture files to hide obscene material.

Separately, the Home Office wants to introduce more sophisticated passport systems and is believed to be working on computer chip containing biometric information such as iris pattern, hand geometry and fingerprints.

The technology could be in use within four years.