More than a quarter (27 percent) of homes and businesses in the UK can't receive 3G signal for all of the UK's five major networks, says Ofcom.
The research is part of the regulator's first 'Communications Infrastructure' report which must be submitted to the Secretary of State for Culture, Media and Sport every three years.
The highlands of Scotland and mid-Wales, which are both rural with hilly terrain, were named the areas of the UK with the worst 3G signal coverage.
Ofcom also revealed 97 percent of premises and two thirds of the UK landmass can receive a 2G signal outdoors from all four 2G networks. This means around 900,000 UK premises can not access all four 2G mobile networks.
The regulator added that while broadband is available through nearly every copper telephone cable in the UK, 14 percent of residential broadband connections are below 2Mbps. Superfast broadband connection, which Ofcom defines as 24Mbps or above, are now available in 58 percent of the UK.
Ofcom said on average residential fixed broadband customers are using 17GB of data per month, which is the equivalent to downloading more than 11 films per month or streaming 12 hours of HD content from the BBC iPlayer service. However, mobile broadband customers use an average of just 0.24GB per month.
When it comes to Digital Terrestrial Television, nearly nine in ten (89 percent) of homes and firms can access the service, while 91 percent of UK also has access to Digital Audio Broadcast (DAB) radio coverage.
As part of the report the regulator has also created coverage maps that split the UK into 200 areas and give each a colour based on the ability to receive signal in the area, with green indicating the highest signal coverage and red marking the lowest.
"This is our first report to the Government on the UK's communications infrastructure. We hope it will be a useful reference point for interested parties, particularly in the light of the recent Government funding package of £150million to help address mobile not-spots," said Ofcom's chief technology officer, Steve Unger
"Over the next 12 months, we expect there to be continued rapid growth in availability of super-fast broadband services and data use, and we aim to publish an update on this next summer."