The Home Office says all new passport photographs must be of an unsmiling face with its gob firmly shut because open mouths can confuse facial recognition systems.

It issued the new guidelines yesterday, The Sun reports, as part of the move towards biometric passports. The new rules come into immediate effect.

Getting computers to recognise faces is notoriously difficult. The theory sounds simple: the system marks each face it encounters on common reference points - the eyes and the tip of the nose for instance - and compares those with faces in its database.

The reality is that it is actually very difficult to do: the angle of each face to the camera must be close to identical to get a good comparison, extra shadows of a face can throw off the reference points, and it seems, teeth are also an overwhelming challenge. Even once these obstacles have been overcome, the accuracy is often low.

The new guidelines state that photographs must have a strong definition between the face and background; be of the full face facing straight at the camera; show no shadows, and that subjects must have "a neutral expression, with your mouth closed".

The guidelines don't apply to existing passports - so there is no need to run out and scowl at a photobooth immediately. Anyone applying for a new or replacement passport must keep their mouth shut in their new picture.

Blunkett will surely be pleased with the tabloid's report. The Sun report confidently tells its readers that "immigration Service officials will run the passport through scanners which will cross-check them against worldwide crime memory banks" and that "the 'biometric' tests ensure that people cannot use stolen or fake documents".

As easy as that, eh? Gosh. We're very relieved to hear it, and we are sure you are too. ®

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