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The UK's biggest book retailer Amazon now sells more ebooks than hardbacks and paperbacks combined, the company has said.

For every 100 print books sold through the site, Amazon said it sold 114 titles for its Kindle e-reader device.

It added that the average Kindle owner bought up to four times more books than they did before owning the device.

The strong figures have been boosted by titles such as multi-million selling erotic novel Fifty Shades of Grey.

The book has sold more than 31 million copies worldwide, with two million ebooks of the title selling in less than four months.

The figures do not take into account ebook sales on other platforms, such as Apple's iBooks or the website for bookseller Waterstones.

Amazon's figures have also been boosted by a surge in popularity for self-publishing.

The company said there had been a 400% increase in authors using Kindle Direct Publishing since summer 2011.

Among them were some of the site's bestsellers. British author Kerry Wilkinson is one of the world's most successful self-published authors.

He has sold more than 300,000 copies of his work in the past year.

Ian Clark, blogger and co-founder of a group promoting greater use of libraries, said the figures should not be seen as a sign that ebooks were dominating over physical sales.

He said that as Amazon was the only official vendor of books for Kindle - by far the most popular e-reader on the market - it had very little competition in selling titles for the platform.

With print sales, however, Amazon faces competition from many big, established retailers.

"To really judge the state of the ebook market, we need proper sales data so that we can compare across the board," .

Jonathan Ruppin, web editor for bookseller Foyles, said it is a trend the book industry must adapt to deal with.

"The proportion of sales that are taken up by ebooks will continue to increase," he told the ´óÏó´«Ã½.

"At some point they will overtake printed sales across the whole market."

He said while online buying could put smaller businesses at risk, he was optimistic that more traditional bookshops still had an important role.

"There's an awful lot of doom-mongering about the retail book trade.

"But for every worrying story I've heard about an independent book shop struggling, I've heard about a good one flourishing."

The figures were released as Amazon marked the two-year anniversary of the Kindle's release in the UK.

"Customers in the UK are now choosing Kindle books more often than print books, even as our print business continues to grow," said Jorrit Van der Meulen, vice-president of Kindle in Europe.

"As a result of the success of Kindle, we're selling more books than ever before on behalf of authors and publishers.

"And thanks to Kindle Direct Publishing, thousands of self-published authors have also been given an outlet to share their work with the millions of Kindle readers worldwide."

However, despite the e-reader's popularity, the company is still yet to announce plans to sell the Kindle Fire - a tablet computer - in the UK, despite it being on sale for over eight months.