O2 has switched on its 5G network, the last of the UK's mobile operators to do so.
The network will initially be available in five cities - Belfast, Cardiff, Edinburgh, Leeds and London.
The firm is targeting key locations such as stations, shopping centres and stadiums, including Twickenham and Arsenal's Emirates ground.
It plans to offer 5G at the same price as 4G, although this will exclude the monthly handset cost.
Slough, the town where O2 is headquartered, will be the first non-city to benefit from its 5G, with 14 other towns and cities to follow this year.
Mark Evans, chief executive of Telefónica UK, which owns O2, said: "We're launching with a range of tariffs that make it easy and fair for customers to access 5G, with flexible plans that cost no more than 4G. We're also switching on 5G in important parts of towns and cities first, places where it will benefit customers and businesses most."
O2 offers four 5G-enabled phones, three from Samsung and one from Xiaomi.
Vodafone and EE have already launched their 5G networks, and last week BT Mobile also put its service live, piggy-backing off EE's network, which it owns. Three has launched a limited service in London, targeting homes.
Paolo Pescatore, a mobile analyst at PP Foresight, said he doubted the fact that O2 was the last to launch would have a detrimental effect. But, he added, all the operators face a challenge.
"All providers have now launched 5G, but the key will be who can make money. This is where the opportunity resides to differentiate, and operators will need to focus on enterprises.
"This will help O2 stand out as it has been working closely with numerous companies."
These include ITN, Samsung and Northumbrian Water Group, and, according to O2, will "demonstrate real-world 5G use cases".
Next year, when the new iPhone is launched with 5G support, will be a key year for the technology, Mr Pescatore added.
While 5G is currently available in dozens of cities in the UK, coverage remains limited for the time being.
BT offers a 5G checker for people to see if it is available in their area.