The bureau says that the faulty notes in circulation are indistinguishable to the naked eye from normal 1000 yen notes, but is remaining tight-lipped about the precise nature of the error, in an effort to prevent forgery.

The faulty banknotes were printed at the bureau’s Takinogawa factory in Kita Ward, Tokyo, in September and October, of last year and were released into circulation in January and February of this year, after they were delivered to the Bank of Japan.

The Bank of Japan says that despite the problems experienced by users of the notes in some electronic machines, it would not be recalling the faulty ones..

“The bills have no problem in terms of quality: they just can’t be used at some vending machines. They’re valid banknotes,” a Bank of Japan official says.

Admitting that the banknotes were defective, bureau director, Minoru Kimura said at a press conference called in Tokyo on Tuesday: “I sincerely apologise for causing trouble to the public and the Bank of Japan.”

All the bureau has said about the error is that it was caused by a combination of a printing machine malfunction and some problems in the printing process. The error was not detected during inspection of the notes.

Japan’s 1000 yen banknotes were redesigned and reissued in 2004 along with other denominations that employed sophisticated security features to fight counterfeiting.

The anti-counterfeiting drive in 2004 called for new designs featuring holograms, watermarks and latent images and were the first major overhaul of Japanese currency in 20 years.

Prior to the introduction of the new banknotes, counterfeits had been easy to use in many of Japan’s ubiquitous 1.8m vending machines, but after the introduction of notes employing the new security features, the number of fakes detected more than halved from 25,858 in 2004 to 12,203 in 2005.

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