Reader Store launched in the US and Canada in 2006, and expanded to Japan in December 2010. It was initially pegged to arrive here last October, but was pushed back to this spring. Sony did not give a reason for the delay.
Previously, owners of Sony Reader devices in the UK had to purchase books from other stores, such as Feed Books and the Gutenberg Project.
But now the Sony Reader Store is available on Mac, PC and Android, offering a wide selection of electronic books, newspaper and magazines.
Around 100,000 eBooks are available to purchase, with prices ranging from 99p to £12.99 for newer releases. Available newspapers include The Guardian and Daily Mail, offered individually or via monthly subscription.
In a statement last September coinciding with the launch of the new WiFi-equipped Sony Reader device, Sony executive Fujio Noguchi said: "Expanding Reader Store will allow us to offer a new, convenient way for readers in all countries it serves to discover and enjoy the books and periodicals they want to read.
"We will of course continue to enhance existing Reader Store services, while simultaneously introducing it in new markets, to deliver a compelling digital reading proposition as widely as possible."
The Reader Store UK can be accessed through any internet browser, but eBooks can only be downloaded through a dedicated application for PC and Mac, along with Android.
This means that books cannot be purchased from the site and then sent directly to a Sony eReader, as with Amazon's market-leading Kindle range.
But all books on the Reader Store UK are made available in the industry-standard ePub format, meaning they can be viewed on various devices other than Sony eReaders, as long as they support Adobe's DRM.
The arrival of Sony's eBooks store follows a law suit lodged by the US government against Apple and five book publishers over their alleged collusion to fix the prices of electronic books as a way to combat the dominance of Amazon.
Apple denies the claims, branding them "simply not true".