UK mobile phone operator O2 has slashed the price of the Samsung Galaxy S2 to under £400, which is not only cheaper than the iPhone 4 but more than £100 cheaper than its SIM free version at Expansys.

The deal, which is available directly from O2 here, comes with a major string attached though, you won't be able to unlock the phone for at least 90 days. However you can still use the phone with O2, Tesco Mobile or Giff Gaff, which amongst them have some of the best deals on the market.

The Samsung Galaxy S2 was only launched back in February 2011 and is already positioned as one of the main rivals to the forthcoming iPhone 5, which is set to be launched next month.

It has a 4.3-inch WVGA Super AMOLED Plus capacitive touchscreen, a dual core 1.2GHz Exynos system on chip, 1GB RAM, 16GB onboard storage, a two-megapixel front facing camera, an eight-megapixel rear one and is the world's slimmest phone at only 8.49mm thick.

The phone is available on any of the five PAYG tariffs currently offered by O2, which means that you will have to fork out at least an additional £7.50 topup for the first month.

That said, there are already rumours about a potential S3 handset coming later this autumn which will be pitted by Samsung against the iPhone 5.