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Over 1,300 years, Japanese writers have created one of the most subtle and delicate of the world's literatures, from 17-syllable haiku poems to the surreal fictions of the contemporary novelist Haruki Murakami. Now the country's literary culture is under siege from that most ubiquitous and modern of devices: the mobile phone.

For the first time, Japan's fiction bestseller list is dominated by books published, read and, in several cases, written on mobile telephones, most of them by young women in their 20s. The rise of the “mobile novel” has prompted an anxious debate about the nature of literature and the future of reading in Japan.

This week the 2007 bestseller list, published by Japan's biggest book distributor, Tohan, revealed that five of the year's most successful novels, including the top three, were first written for downloading on mobile phones before being republished in book form. The number one seller, Love Sky, sold two million copies in the last year, has recently been released as a hit film, and has made a star of its author, a woman in her early 20s known only as Mika.

A sequel, Your Sky, came in at number three, and second place went to Red String by Mei, which sold one million copies. All are written in short, simple sentences using relatively few characters, featuring melodramatic plots heavy on violence, sex and tear-jerking sentiment. Love Sky, for example, tells the story of a teenage girl who is bullied, gang-raped, becomes pregnant and suffers a miscarriage.

“The fact that young readers are being exposed to immature expressions and stunted vocabulary will accelerate illiteracy and damage their ability to express themselves,” one literary critic wrote. But others believe that the new genre is doing literature a service by promoting reading among young people who would otherwise have little interest in books.

The new dominance of mobile novels - keitai shosetsu in Japanese - is all the more remarkable for the speed with which it has come about. They did not exist in 2002, but the following year online sales were worth 1.8 billion yen (£8 million). By 2006, the figure has risen to 9.4 billion yen (£42 million). They owe their success to the popularity of the mobile phone among young Japanese, who were taking photographs, surfing the internet and sending emails on their keitai long before their peers in the West.

Several publishers operate mobile novel websites from which phone users can download novels for a subscription of about 300 yen (£1.33) a month. The stories are divided into gobbets which can be read in about three minutes, the typical distance between two stops on the Japanese subway.

Only a fraction of these have become hits, but those that have share a style: short sentences (essential on mobile phone screens which hold only 100 characters), lots of dialogue and a distinct absence of the lengthy descriptions which characterise more traditional Japanese fiction.

The principal characters of the stories, like their readers, tend to be young city dwellers. Tragedies, in the form of bullying, rape, murder and infection by HIV, strike them with terrible regularity. “The typical storylines of keitai shosetsu are corny,” says the sociologist Kensuke Suzuki. “It is a world of right and wrong, and is quite un-literary.”

This, of course, is their appeal. “High school students experience mobile phone novels as real life,” writes the high-brow literary journal, Bungakukai, which features keitai shosetsu in its latest issue. “For these readers, they are a substitute for pop music and comics.”

Extract from Love Sky by Mika: “I'm short, I'm stupid, I'm not pretty, I'm rubbish, and I've got no dreams.”

From Red String by Mei: “Since the 8th grade I have had many experiences that I shouldn't have had and learned things I should never have learned. I have been soiled.”

From If You by Rin: “I don't have a girlfriend, but I fancy the girl next door. She doesn't have a boyfriend or any friends at all, as she is rather gloomy. This is a story about us.”

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