US etail giant Amazon has revealed that sales of ebooks have overtaken those of paperback physical books for the first time.

In its fourth quarter, for every 100 paperbacks Amazon sold in the US, 115 digital Kindle ebooks were sold.

Last summer the retailer revealed that sales in ebooks had overtaken sales in hardback books. In the fourth quarter this trend continued, with Amazon selling three times as many Kindle books as hardcover books.

Amazon net sales increased 36% to $12.95bn in the fourth quarter, while net income increased 8% to $416m.

In the full year to December 31, net sales increased 40% to $34.2bn. Net income increased 28% to $1.15bn.

However, sales were not as a good as expected and Amazon shares fell 9% in after-hours trading as a result.

In its international arm – comprising UK, Germany, Japan, France, China and Italy – sales surged 26% to $5.74bn in the fourth quarter.

Founder and chief executive of Jeff Bezos said: “Thanks to our customers, we achieved two big milestones. We had our first $10 billion quarter, and after selling millions of third-generation Kindles, Kindle books have now overtaken paperback books as the most popular format on

“Last July we announced that Kindle books had passed hardcovers and predicted that Kindle would surpass paperbacks in the second quarter of this year, so this milestone has come even sooner than we expected - and it’s on top of continued growth in paperback sales.”