Crowds are expected to flock to Apple stores throughout the country to get their hands on the latest must-have smartphone.
Described by its makers as "the best iPhone ever", the iPhone 4S is set to fly off the shelves when shop doors open.
Crowds of technophiles are expected to stream into the company's flagship outlet in central London, where queues snaking down Regent Street are a familiar sight when new products go on sale.
The iPhone 4S has already proved itself to be more popular before its release than any other Apple product, with pre-orders topping one million in a single day.
New features include a camera with advanced optics; Siri, a so-called intelligent assistant that "helps you get things done just by asking"; and iCloud, a service which synchronises content across multiple devices.
Every customer who buys an iPhone 4S at one of the UK's 33 Apple shops today will be offered a free set-up service to help them customise their phone and get going on email.
The device goes on sale just nine days after the company's pioneering former chief executive Steve Jobs died following his lengthy battle with pancreatic cancer.
And it comes after iPhone's rival, BlackBerry, sparked widespread anger following a technical failure which left millions of its customers unable to access email or internet services for more than three days.