O2 customers will be able to access free WiFi on London's Tube network from June.
A deal with Virgin Media, who operate the internet service on the London Underground, means that subscribers will be able to get online at 100+ stations around London.
The service was initially offered for free in 2012, starting with the London Olympics and continuing until the end of January. From then the service was offered to customers of Virgin Media and other networks who signed up to the program.
O2's deal is similar to that struct by EE and Vodafone, and will see the service be offered for no extra cost to both pay monthly and pay-as-you-go customers.
Three is now the only large network not to offer free Tube WiFi to its users.
The news coincides with 12 more stations gaining WiFi access this week - Acton Town, Baker Street, Bank, Caledonian Road, Earl's Court, Holland Park, Ladbroke Grove, Maida Vale, Queen's Park, Sheperd's Bush, Sloane Square and West Ruslip.
Gary Booke, head of 'consumer' at O2, said:
"We're excited to bring free Underground WiFi to O2 customers this summer. Our customers want to stay connected wherever they are and will soon be able to check their email, update their Facebook status, shop online, book their next holiday and stay connected to their friends and family right across Tube stations in Central London."