The rest of the UK is lagging behind London when it comes to accessing 4G data services, according to a report from mobile coverage company OpenSignal and consumer watchdog Which?
While mobile users in London can access 4G nearly 70% of the time, in Wales users can connect only 35% of the time, the findings suggest.
Regulator Ofcom receives a large volume of complaints about mobile coverage.
All the mobile companies said they were investing in mobile networks.
The State of the Mobile Network report found that EE had the greatest availability of 4G networks, on 64%, with Vodafone on 60%. 02 was slightly behind on 59.9%. And 3 offered the least availability, with 43.7%.
When it came to average download speeds, EE was again in first place with, 27.9Mbps on average, followed by 3, on 24.4Mbps, Vodafone, on 17.9Mbps and O2, on 16.1Mbps.
Although London has the most availability, the speeds there were not as high as in other regions, due to the number of users attempting to use the network.
"It's clear mobile providers must do much more to improve their networks if they are to provide greater access to their customers," said Alex Neill, Which? managing director of home and legal.
An earlier report from OpenSignal and Which? found that users in the UK were able to access 4G about half of the time on average.
"Finding a 4G signal outside of London can still be a bit tricky," said Brendan Gill, co-founder of OpenSignal, but he added that mobile data speeds were "consistently fast".
Regulator Ofcom is currently compiling a crowd-sourced coverage map and calling on the UK's 20 million Android users to help build it by downloading an app that automatically measures signal strength and data speeds.
In response to the report, Vodafone said: "Thanks to our £2bn investment in our network and services since 2014, with another £2bn expected over the next three years, our customers across the country are experiencing a significantly improving and stronger network every day."