The mobile network O2 has started testing a next-generation mobile data service in London.
The nine month trial will involve hundreds of consumers and businesses using a Long Term Evolution (LTE) network across parts of the city.
O2 says its LTE downloads will be many times faster than those over 3G.
The news comes a fortnight after MPs urged O2 and other operators to stop fighting over how 4G airwaves should be shared out.
The experiment is an extension of O2's LTE trials in Slough, west of the capital, which began in 2009.
Participants will be given broadband dongles for their computers allowing them access to the 2.6GHz spectrum.
When the network goes into full-time service it will also include the lower frequency 800MHz band which will be freed-up by the UK's switch from analogue to digital TV.
Members of the trial are being promised download speeds of up to 50 megabits per second (Mbps), although O2 says the network can theoretically transfer data three times faster.
Once the network is rolled out across the UK the firm says speeds are more likely to average 10-15Mbps.
By comparison telecoms watchdog Ofcom said in May that the UK's average 3G download speed was 1.5Mbps.
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," said Ronan Dunne, chief executive of Telefonica UK which runs O2.
"The work we are doing now will lay the foundations for our commercial 4G network when it launches in the UK."
O2 said it expects more than 1,000 people to take part. That makes the test bigger than a separate LTE trial being carried out by Everything Everywhere and BT in Cornwall involving 200 residents.
Ofcom plans to hold an auction for the LTE radio spectrum towards the end of 2012. However, because the spectrum will not become free until 2013 it will be some time after that before the services can be offered on a commercial basis.
That puts the UK behind other countries including Sweden, South Korea, Germany and the US which have already begun their rollouts.
Earlier this month the Commons Culture, Media and Sport select committee said it was concerned that disagreements among the UK's mobile network operators had appeared to have delayed the auction's process.
O2 and Vodafone are threatening legal action if Ofcom allows Three to buy part of the LTE spectrum at a minimum price. Ofcom wants to ensure there is competition in the sector, but O2 and Vodafone claim the move amounts to state aid.