Middlesbrough is the best place in the UK to get mobile 4G coverage while Bournemouth is the worst, says a report.
Consumer group Which? and analyst OpenSignal measured data from mobile phones across 20 cities in the UK.
They say "critical" reforms are needed to provide a better service for customers.
Ofcom said its rules meant "virtually all" UK premises would have to receive a 4G signal by the end of the year.
The OpenSignal study analysed more than 500m data readings from mobile phones taken from more than 30,000 users between December 1 2016 and February 28 via an app.
It ranked 20 of the biggest towns and cities from top to bottom based on their 4G availability.
- Middlesbrough/Teesside - 82.7%
- Sheffield - 79.3%
- Sunderland - 79%
- Leicester - 78.6%
- Leeds/West Yorkshire - 78.2%
- Bournemouth/Poole - 67.5%
- Southampton/Portsmouth - 69.6%
- Cardiff - 71.8%
- Nottingham - 73.3%
- London - 73.6%
The report also looked at average 4G download speeds across the UK, finding Stoke-on-Trent to be the fastest city and Brighton the slowest.
Which? says big cities often suffer from sub-par mobile networks because it is more difficult to build towers and masts in built-up urban areas.
The findings will be of particular concern to businesses which see good mobile connectivity as vital to a competitive economy, says BBC technology correspondent Rory Cellan-Jones.
The research will also lend weight to the argument that it is far too early to focus on 5G when the operators still need to invest in the infrastructure needed to take 4G everywhere, he added.
Which? is calling on the next government to work with Ofcom and mobile providers to ensure "critical reforms" are made to ensure a better performance and service for customers.
Alex Neill, Which? managing director of home products and services, said Ofcom needed to "keep the pressure" on mobile operators so every part of the country got a "decent service".
"Our mobile phone is central to how we live our lives and that is why it is so frustrating when we can't access emails or browse the internet on the go," she added.
Ofcom said it agreed mobile coverage must improve and that it understood the "importance" of having a reliable mobile broadband where people live and work.
A spokeswoman said: "Ofcom rules mean that virtually all UK premises must receive a 4G signal by the end of this year.
"We're also making available valuable new airwaves to boost mobile broadband, and have challenged mobile operators to explore how to reach all remote areas and transport lines."