The rest of the UK is lagging significantly behind London for mobile 4G access and providers must do "much more" to improve their networks, researchers say.
Mobile users in London can access a 4G signal 69.7% of the time but those in Wales - the worst region for accessibility in the UK - can only connect 35.4% of the time, consumer group Which? and independent mobile coverage analyst OpenSignal found.
The second best area in the UK for 4G access is Yorkshire and Humberside, where mobile users can connect 61.3% of the time.
But, although London is the best area for 4G connections, it has the lowest average for 4G download speeds of all of the 12 UK regions in the study.
The average download speed in London is 18.8Mbps, compared with Northern Ireland's 23.3Mbps.
The North East is the second worst area in the UK for 4G download speeds with an average of 20.1Mbps.
A performance review of providers Three, EE, O2 and Vodafone by OpenSignal ranked EE the best for both 4G speed and accessibility.
Three was the worst performer, with users only able to access 4G on average 44% of the time.
Which? managing director of home and legal Alex Neill said: "This latest research confirms that the rest of the UK is lagging behind London when it comes to accessing 4G signals.
"It's clear mobile providers must do much more to improve their networks if they are to provide greater access to their customers."
Brendan Gill, the chief executive and co-founder of OpenSignal, said: "There was a lot of good revealed in our data about UK operators. Mobile data speeds, both 4G and overall, were consistently fast in our tests throughout the country.
"But there definitely are still some areas in which the UK needs to improve. Finding a 4G signal outside of London can still be a bit tricky."
:: OpenSignal and Which? examined 500 million tests conducted by more than 28,000 smartphone users between June 1 and August 31.