Cinema ushers across Britain go into action today with a new piece of equipment which makes their ice-cream trays and hand torches look tame.

Military-style night-sights have been sent to every outlet in the country showing the new Harry Potter film, The Prisoner of Azkaban.

Staff have been instructed to spend all two hours and 22 minutes of the film scanning the dark - for pirates making illegal copies.

"I''ve never known a company to go to such lengths to protect a film," said Jamie Graham, manager of the Vue cinema at Cheshire Oaks, Wirral, where the red monocle devices are ready for action.

The precaution has been taken by the film''s distributor, Warner Brothers, after an epidemic of poor-quality, grainy versions of the two previous Potter films.

Surreptitious recording from cinema seats, sometimes interrupted by the head of the person in front shifting and blocking the action, has become a serious menace, according to the industry.

Most cinemas now screen an appeal to audiences to shop any neighbour suspected of filming, along with warnings about mobile phones and adverts for popcorn.

Mr Graham said: "Video piracy is rife everywhere, and with the UK screening the film four days before the rest of the world, Warner was concerned the movie would end up on the internet."

Pirate DVD versions of the boy wizard''s earlier adventures were traced to Britain through codes imprinted on the films as a security device.

The night sights, together with the coding and experiments with watermarks, have added significantly to distribution costs. But Warner sees the investment as negligible compared with the threat to the whole industry.

Staff at the Vue will be "very discreet" with their potentially frightening cyclopean attachments, Mr Graham said, but action against offenders would be swift.

Much like the battered young wizards on screen, who are constantly being whirled about by baddies, pirates will be "hauled out of their seats and reported straight away to the police".