Apple customers are expected to be able to download the company's latest operating system within hours - which has been hailed as a new beginning for the technology giant.
British design chief Sir Jonathan Ive introduced iOS 7 as an "important new direction" when he showcased the software earlier this year.
Based on previous iOS releases, many have predicted that the release time will be 6pm UK time.
The new system has a cleaner look than its predecessors and Apple's CEO Tim Cook said it was "the biggest change to iOS since the introduction of the iPhone".
It was unveiled just months after Apple posted its first profit slide in a decade and drew accusations that it had failed to innovate.
But its launch comes just days before two new iPhones go on sale - one of which features a fingerprint scanner.
Executive Phil Schiller sent a massive cheer through the audience at the San Francisco conference in June when he told developers: "Can't innovate any more, my a***!"
Industry observers appeared largely convinced and suggested the arrival of iOS 7 could go some way to silencing Apple's critics.
The new software has been designed to make the iPhone appear bigger, with features crafted to take advantage of the entire screen.
Text is said to appear sharper, while a "control centre" on the phone allows users to adjust settings with just one upward swipe.
This gives instant access to functions such as Airplane Mode, wi-fi, Bluetooth or Do Not Disturb, and enables users to quickly pause or play a song, jump to the next track and stream music.
