They include Airvana, an arm of the US networking giant Commscope, which specialises in so-called “small cell” technology that allows mobile signals to operate in tight spaces.
Plans to bring mobile coverage to the London Underground network form part of a nationwide overhaul of communications for the emergency services.
A bespoke radio network, Airwave, will be replaced by standard 4G mobile technology, saving hundreds of millions of pounds a year and providing high-speed internet access to police, paramedics and fire crews in the field.
The switch-over is due to begin next year and to be completed by the end of 2020, with the new signal to be provided by BT’s EE mobile network.
London is the last region due to be switched over to the new system, in part because of the challenges on the Tube network.
An industry source involved in discussions with TfL said it was not yet clear how mobile coverage will be delivered underground.
He said: “It could be controlled by TfL, by one of the operators, or by some independent infrastructure provider. It could be piggybacked on some of the Wi-Fi infrastructure that is already down there.”
Another source said: “The heart of this is the emergency services network, but once it is done, the Government want commercial mobile operators to connect the Tube.”
A TfL spokesman confirmed officials were investigating how to introduce mobile coverage on the Tube, initially for the emergency services.
He said: “We do not currently have plans to introduce mobile phone coverage for customers. The introduction of such coverage would need to be commercially viable and would be subject to consultation.”
The London Underground gets a new managing director this month, Mark Wild, former chief of an Australian transport authority.