Existing customers will be able to use 5G at no extra cost - if they have a compatible handset - while new contracts will include 5G as standard.

One analyst said Three's pricing had "undercut" rivals Vodafone and EE.

Both have already launched a 5G service in some major cities but Three will not launch its service until August.

As well as faster download speeds, one of the benefits of 5G is increased network capacity. In theory, it should mean an end to network congestion in busy areas such as pop concerts or festivals.

EE launched its 5G service in May. Currently its cheapest Sim-only 5G tariff costs £32 a month, but it comes with a 20GB data download cap.

Vodafone offers a Sim-only 5G tariff for £23 a month, but limits the download speed to 2 Mbps, making it slower than 4G. It charges £30 a month for true full-speed 5G.

Three says it will offer unlimited data without a speed limit for £22 a month, the same price it currently charges for its 4G tariffs.

The network told the BBC there was "absolutely no throttling".

"Our 5G is completely unrestricted and there no catches," a spokeswoman said.

On Monday, Three was criticised after it refused calls from the industry regulator Ofcom to automatically cut its customers' monthly charge at the end of their contract's lock-in period.

As a result, Ofcom said Three's subscribers would "overpay" unless they took action to change to another deal.

"The UK is rapidly becoming the most competitive market for 5G and Three's announcement raises the ante once again," said Kester Mann, an analyst at the CCS Insight consultancy.

"It is surely only a matter of time before all four UK operators offer unlimited 5G data."

Mr Mann said 5G was "crucial" for Three because it provided "an opportunity to reinvigorate the brand, overcome a negative network perception [and] achieve the scale it has long-for craved".

But he warned that the price war made it "increasingly challenging for operators to make a significant return on the huge investment required".

O2 will be the last of the major networks to launch a 5G service. It plans to switch on its offering in October.

It will be the only network to offer 5G without using equipment from the embattled Chinese telecoms equipment-maker Huawei. O2 has not yet announced its pricing.