O2 has added London to the list of cities that can now enjoy faster 3G access with its mobile phone network.

O2 has been repurposing the old 900MHz network for 3G after Ofcom agreed earlier this year that it was no longer required for 2G mobile phone use. According to O2, this will provide a 50% increase to its 3G network capacity and, it’s reckoned, will allow provide customers with up to 30% faster 3G access.

The only catch is that a mobile device with 3G900 support is required to make use of 900MHz 3G network. Many new smartphones do support 900MHz 3G (usually listed in their specifications under “Network”), many relatively new ones do not. So, while the iPhone 4 and HTC Desire can make use of 3G900, for example, the iPhone 3GS cannot. The same applies to 3G USB modems (‘dongles’).

London joins a number of cities in the UK that now offer 3G900, including Leeds, Manchester and Birmingham. There’s no news yet on if or when O2 will use the 900MHz network to expand its 3G coverage in rural areas, though

Nigel Purdy, Chief Technology Officer for Telefónica O2 UK, said: “This 3G900 switch-on is the latest in a series of network enhancements for O2, and is a first for the UK and Telefónica. We’ve acted quickly to bring the benefits of 3G over 900MHz spectrum to our customers and we’re thrilled with the results. Quite simply, on O2’s network, more customers can use more data, and experience it quicker.”