According to tests by mobile network monitor RootMetrics, Vodafone has the edge on O2 in London, recording download speeds of 9.5 Mbps and upload speeds of 4.7 Mbps compared to O2's 7.1Mbps and 3.7 Mbps. Previously, O2's average download speed was 3.2 Mbps and its upload speed was 1.2 Mbps, while Vodafone's was 4.8Mbps and 1.4Mbps.
"The results of our 86,225 independent tests across London show that O2 and Vodafone have made a great start with their 4G networks, and consumers should be enjoying notably faster speeds. Certainly compared to other rollouts of 4G we have seen, London has got off to an impressive start," said RootMetrics CEO Bill Moore.
"With more 4G masts lighting up every day, we are seeing increasing speeds in the capital as time goes on," he added.
However, EE, the UK's largest mobile operator, remains the fastest and highest rated overall network.
Overall, EE took top place in the RootMetrics' tests, with a score of 90.6. O2 came second with a score of 81.4, while Vodafone and Three tied third, despite slightly different scores of 72.3 and 71.4 respectively.
EE launched its double speed 4G network network earlier this year and has recorded "exceptionally fast" average speeds of 19.3 Mbps (download) and 12.9 Mbps (upload). According to RootMetrics, its upload speeds are up to 249 percent faster than O2 and 174 percent faster than Vodafone. RootMetrics points out that EE's speeds are nearly twice as fast as the average fixed-line broadband, which Ofcom puts at 12 Mbps.
EE's fastest recorded download speed was a single test that hit 90.5 Mbps on Eastern Avenue in Ilford, North East London. That is 32.5 percent faster than Vodafone's top download speed of 68.3 Mbps recorded in Ashford, Surrey, and 40.7 percent faster than the top download speed of 64.3 Mbps that O2 recorded just off Piccadilly Circus.
Three of the four major networks in the UK now offer 4G.
However, that will change when mobile network operator Three joins Vodafone, O2 and EE in December when it launches its own 4G service. At present, Three's 3G download speeds are only capable of reaching 5.4Mbps and its upload speeds max out at 1.6Mbps.
The tests also revealed that people living in London's surrounding areas such as St Albans, Maidstone and Basildon could expect uploads and downloads to take nearly twice as long as if they were in Zone 1, in the centre of London.
Vodafone's mobile internet presented more difficulty with successfully making and keeping connections, which ultimately bumped down the network's score. When it came to mobile internet accessibility, Vodafone placed lower (88.0 percent) than the other major networks; EE (95.1 percent), Three (93.8 percent) and O2 (90.4 percent).
But RootMetrics points out that not everyone cares about mobile internet. "If this is you then O2 a good choice," it said, pointing out that O2 recorded the fewest call failures of any network, and tying with EE for top placement the text category.
RootMetrics has an app that measures network coverage, data speeds and call success and failure rates for all of the major operators across all major cities and towns. The app aims to help consumers view coverage near them and upload their own test results whether they are in an area that has been professionally tested or not. The information collected by users of RootMetrics is put into online maps to help mobile phone users make decisions about the mobile phone company that's right for them.