Following a successful trial during the summer period on the Waterloo and City line, TfL is sticking to its promise to begin rolling out full 4G coverage on the London Underground network from 2019.

The trial, which included design input from UK mobile operators Vodafone, O2, Three and EE, saw a phone call being made from one station to another without dropping reception. The trial was carried out outside customer hours, and so allowed TfL to have time practicing and figuring how best to lay fibre cables, which will be instrumental in the overall plans for 4G coverage.

All test equipment and fibre cables from the trial have since been removed from stations and tunnels, suggesting the very latest equipment available closer to roll out will be used when the plans kick into action.

Tendering to find a service provider to take on the contract will go out in the New Year.

London Mayor Sadiq Khan pledged back in August that the Underground would catch up with other underground transit systems in Paris, Tokyo and New York and offer full mobile coverage throughout the Underground network, so customers can call, message, Instagram, Facebook and Snapchat friends and family without having to worry about signal dropouts.

The plan is for customers to have mobile signal throughout their entire journey on the trains, including in tunnels, rather than having to reply on Wi-Fi at the majority of stations.

Graeme Craig, Director of Commercial Development at TfL, said: "The success of this trial shows that we are on track to unlock one of the UK's most high profile not-spots and deliver 4G mobile coverage throughout our tunnels and Tube stations."

"This is great news for our customers and will also help us generate vital commercial income to reinvest in modernising and improving transport in London."