Consumers were warned about the possibility of their iPods overheating yesterday after Japanese authorities reported three incidents of the devices catching fire.

Apple, which makes the iPod, confirmed yesterday that older versions of the iPod Nano could overheat, but stressed that such incidents were extremely rare and occurred in "less than 0.001 per cent" of devices.

The Japanese Government said it was investigating a possible battery defect in the iPod Nano after it received three reports of the device overheating - two in Tokyo and one in the Kanagawa Prefecture, near the capital.

In the reported incidents, the devices sent out sparks which scorched nearby paper and in one case set fire to a woven straw mat, according to an official in the Japanese Ministry of Economy, Trade and Industry.

Apple said in a statement that "in very rare cases", batteries in 'first generation iPod nanos' which were sold between September 2005 and December 2006 had been known to overheat, and that it would replace any faulty devices.

It added: "There have been no reports of serious injuries or property damage and no reports of incidents for any other iPod Nano model." Customers worried about a device they owned were advised to contact AppleCare services.

According to the Japanese ministry, the affected iPods caught fire while they were being recharged.

The Nano - a smaller version of the Apple's top-selling music player - gets its power from a rechargeable lithium ion battery. These are used by several manufacturers of personal computers and other electronic devices and have been known to overheat.

Last year Matsuhita, owner of the Panasonic brand and one of the world's largest producers of lithium ion cells, recalled 46 million batteries that were made for Nokia phones after they were found to overheat once they had been charged a couple of hundred times.

Several senior Japanese researchers have called on manufacturers to overhaul the way they produce the batteries, saying the design means that the cells are fundamentally unsafe.

But production lines of the lithium ion cell - which was first introduced in 1992 - are relatively young, and companies are reluctant to redesign a technology that has already required substantial investment.

Between them, Sony, Sanyo and Matsuhita - the world's three largest producers of the batteries - have spent billions of yen on product recalls in recent years.

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