Londoners will be able to use their phones to make calls or go online anywhere on the Tube network by the mid-2020s, TfL has announced.
Jubilee Line passengers will be the first to benefit from the roll out of the 4G network on the Underground, with the eastern part of the line to get connectivity by next March.
The trial section will stretch from Westminster to Canning Town - excluding London Bridge and Waterloo stations - and cover platforms, tunnels, ticket halls and corridors.
TfL pledged on Friday that by the mid-2020s the entire Tube network will have connectivity.
Mayor of London 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.
"London is the best place to live, visit and work - and projects like this will help make it even better."
There are already 260 wifi-enabled stations on the London Underground and on TfL rail services.
Shashi Verma, chief technology officer at TfL, said: "The London Underground network is an incredibly challenging environment in which to deliver technological improvements, but we are now well on the path to delivering mobile connectivity within our stations and tunnels.
"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."
The entire project is expected to require 2,000 kilometres of cabling with engineers working week-night shifts to minimises disruption to passengers.