The iPhone 3GS, the S standing for speed, was announced yesterday at Apple's Worldwide Developers Conference in San Francisco.
The phone features faster loading times, more memory, longer battery life, an upgraded video-capable camera and voice activated controls.
O2 will retain sole carrier rights in the UK as it did with the iPhone 3G.
The 16GB iPhone 3GS will be free on O2's highest monthly tariff rates (£44.05-£73.41 per month), with a requisite 18 or 24-month contract.
With the lowest monthly tariffs on an 18-month contract, the 16GB iPhone 3GS will cost £184.98, or £274.23 for the 32GB model.
O2 customers with the older 3G model will be able to upgrade to a 3GS when their contracts expire, or buy out their current contract, which could potentially cost hundreds of pounds.
Disgruntled O2 customers have flooded online message boards and Twitter to express their anger at the upgrade costs.
Apple also announced that it will cut the price of the iPhone 3G to $99 (£61) in the US, however O2 has not said if it would carry the price cut over to the UK.
The discounted 3G is seen as a strategic manoeuvre for Apple, which is timing its move soon after the launch of the Palm Pre, a new smartphone being touted as the first worthy competitor to the iPhone.
The Palm Pre, which was released two days before the iPhone 3GS, has reportedly sold over 100,000 in 48 hours, less than half as much as the iPhone did two years ago.
The iPhone release date also coincides with Nokia's launch for its new smartphone, the N97.
Nokia is hoping to capitalise on the success of its N95 handset, which has sold 10m units worldwide.
Apple said it has sold over 40m iPhones and iPod Touches since its launch in 2007 and has over 50,000 applications available in its App Store.
At the WWDC conference Apple also upgraded its line of MacBook laptops, cutting prices, increasing speed and battery life.
The conference was helmed by Apple's marketing boss Phil Schiller in the absence of CEO Steve Jobs, who had been rumoured to show after taking leave for persistent health issues.