Passengers will be able to make calls and go online anywhere on the London Underground by the mid-2020s.
Transport for London (TfL) has announced plans for mobile users to have 4G access across the Tube network.
The Jubilee Line will be the first to benefit, with the eastern half to get full mobile connectivity on both platforms and tunnels from March.
There are already 260 wi-fi-enabled stations on the London Underground and on TfL rail services.
Shashi Verma, chief technology officer at TfL, said: "We have begun the complex work to allow our customers to be able to get phone reception within our tunnels from March 2020, with more stations and lines coming online during the coming years."
London Mayor Sadiq Khan said: "This is a really important step for the millions of people who use the Tube each year.
"Introducing 4G and, in the future, 5G, will help Londoners and visitors keep in touch and get the latest travel information while on the go."
The trial section on the Jubilee Line will stretch from Westminster to Canning Town - excluding London Bridge and Waterloo stations - and also cover ticket halls and corridors.
London Bridge and Waterloo stations are due to become fully 4G enabled later during 2020 subject to final approval, TfL said.
The entire project is expected to require 2,000km (1,242 miles) of cabling with engineers working weeknight shifts to minimises disruption to passengers.
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