The Airwave radio network has been rolled out on the London Underground, enabling police to use radios underground.
Police say it will enable them to operate much more effectively during incidents such the 7 July bombings in 2005.
The roll-out of the system to 125 tube stations came after the London Assembly recommended it be done in the wake of the bombings.
Police forces have had the ability to communicate using Airwave above ground since 2005. It uses Tetra technology, which is an open digital trunked radio standard.
The system is separate from mobile phone networks, which means the emergency services can still communicate when high volumes of calls around an accident scene are blocking networks.
Communications over the system are encrypted, with digital sound quality and background noise reduction.
Police officers can now access the Police National Computer (PNC) through handheld devices while underground. The PNC holds details on suspects as well as a range of public information and was accessible only via police stations before the Airwave system was rolled out.
Police Minister Vernon Coaker said, "The Airwave system plays a vital part in keeping passengers safe," and Tony McNulty, Minister for London added, "One of the key lessons from the London bombings of 2005 was the need to enhance the resilience of responders' telecommunications systems and communication underground. I am, therefore, delighted to see that this system is now fully in place."