Vodafone is deeply unhappy about a new infrared “night filter”, made in Japan, which is fitted to its phones and causes skirts, trousers, shirts and blouses to melt away. Users said it is particularly effective on dark bikinis.

Dubbed the “pervert filter”, the device is made by a Fukui-based company called Yamada Denshi, and can be purchased for less than £100 over the internet.

It is particularly effective when used with high-end camera phones with powerful zooms and autofocus. In Japan, it is frequently fitted to the Vodafone V602-SH.

Although this model is not available in Britain, it is expected to work with similarly high-end models that are available here. The cameras are not illegal, although using them to film people’s naked bodies would be.

The cameras work like a night-sight, picking up infrared radiation — heat — which is given off by the body. The body is hotter than the clothes, so the radiation that you see gives you the outline of the person being filmed.

Vodafone has condemned the abuse of mobile-phone mounted cameras, even though it is a technology which the British company pioneered in Japan.

Vodafone and rival networks in Japan have particularly focused on the potential for invasion of privacy in their “best practice” sermons to customers.

A spokesman for the company said yesterday: “This is a third-party device so our control is limited. Obviously we would never sell a phone that was enabling someone to see someone naked. That would be a contradiction of all our policies and we will do everything to stop such devices spreading.”

The official purpose of the filter is for taking pictures at night. While openly acknowledging the see-through functions of its gadget, Yamada is at great pains to insist that the technology should be used for “academic purposes”.

“We only sold these filters because we expected the night shot function would be used for taking pictures such as a baby asleep in bed, or the fine details of a leaf,” said a Yamada spokesman.

“It sickens me that anyone would be using them to take sneaky shots.”

The technology to see through clothes has always been prohibitively expensive. Now, just about anyone with a mobile camera can do it at minimal expense with these new filters.

The easy-to-fit filter is the latest gadget to have fallen instantly into the hands of the mischievous and obsessive world of underground Japanese voyeurs.

Within the past four months, two professors from the country’s leading universities have been arrested for taking hidden camera pictures up their female students’ skirts.

The technology that can make clothes appear transparent has already caused problems in Japan.

Two years ago, Sony released a video with a more primitive version of the night filter built in. Sony said that the filter was intended to assist camera work in the dark, but quickly became aware of its voyeuristic talents.

Sony discontinued the line and re-engineered its video cameras to prevent further problems.

Meanwhile, Metropolitan Police in Tokyo established a system this year that allows the public to e-mail mobile-phone snaps of suspicious-looking people to a central database.

The illegal activities of Japan’s huge number of sinister voyeurs are a constant source of fear and irritation for many Japanese women. Six members of a club that specialised in “uppu-sukaato” photographs taken of women on trains have recently stood trial in a case that revealed the massive extent of the voyeur network they were part of.

Few Japanese had any sympathy therefore when, in April, a man intent on spying on girls became trapped in the ventilation duct above a women’s lavatory at Kansai International Airport.

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