O2 has started a nine-month trial of 4G mobile data in London. The trial, of the LTE (Long Term Evolution) network, is an extension of an existing scheme that started in Slough in 2009. O2 says it will offer faster downloads to up to 1,000 consumers and businesses. Most of the country will still have to wait until 2013, however.

Those consumers and business will receive broadband dongles that allow them access to the 2.6GHz 4G spectrum. The part of the spectrum used for 4G will be freed up by switching off analogue TV signals. 4G is expected to take over from existing 3G networks, offering faster download and upload speeds. Ofcom is set to hold an auction for the LTE radio spectrum. It has been delayed until late 2012, meaning that commercial 4G networks are unlikely to launch before mid-2013, putting the UK behind countries such as the US, South Korea and Germany, who all have some 4G connectivity.

Legal action from O2 and Vodafone may also slow the rollout. Those ISPs are threatening legal action if Ofcom allows Three to buy part of the LTE spectrum at a minimum price. Ofcom is believed to favour this approach so that lots of telcos can offer 4G services, ensuring greater competition. But Vodafone and O2 believe that such action would constitute state aid for commercial organisations.

In theory, 4G can offer download speeds of up to 150 megabits per second (Mbps). O2 says that its trial should offer real-world figures nearer to 50Mbps, although after national rollout most telcos believe that a realistic figure is closer to 10- to 15Mbps. The UK's average 3G download speed is around 1.5Mbps, Ofcom says.

See also: How to get 100Mb broadband

O2 describes the service as being 4G. However, the International Telecommunications Union (ITU) defines LTE as being 3.9G since it falls short of one gigabit per second transfers.

"Today's launch of the UK's first 4G London trial network demonstrates our commitment to delivering 4G to our customers at the earliest opportunity. The work we are doing now will lay the foundations for our commercial 4G network when it launches in the UK," said Ronan Dunne, chief executive of Telefonica UK which runs O2, in a statement.