Three will launch its first 5G network this August in London, before extending it to 25 other UK towns and cities before the end of the year.
The telecoms company will kick off with a home broadband service in the capital but is also launching mobile services.
It follows BT’s EE and Vodafone in launching next-generation 5G services in 2019.
Three, which is owned by Hutchison, said it would not share details about the cost of its services until July.
Chief executive Dave Dyson said: “It’s clear that consumers and businesses want more and more data.
“We have worked hard over a long period of time to be able to offer the best end-to-end 5G experience.
EE launched 5G services in six cities in May, while Vodafone will launch one on 3 July.
According to some, 5G will offer download speeds as much as 100 times faster than existing 4G networks.
However, there have been concerns about the pricing and connectivity of such services.
Both have also pulled smartphones made by the Chinese company Huawei from their 5G launches, because of uncertainty about support by Google’s Android.
