Plans to bring 4G telecom networks to the London Underground have been hit by concerns of soaring costs for mobile operators, creating a fresh obstacle for transport bosses after more than a year of delay.

The latest proposals are fuelling fears that mobile carriers such as Vodafone, O2 and Three could pay a higher price to access the infrastructure than previously expected, City sources said.

It comes as Transport for London (TfL) prepares to bring 4G mobile access for passengers across the Tube network in 2020, after delaying the project by more than a year.

One of the sticking points has been the expansion of the project to include the Home Office’s new emergency services network on the same signal.

Sources say it is still unclear how expenses will be split between public connections - paid for by a third party provider - and emergency services, which are funded by the Home Office.

There have also been questions around the business case for connecting the overground sections of TfL’s network, which are likely to carry less traffic than London’s busy centre.

While the majority of the Tube’s tunnels are concentrated in the centre of London, nearly 80pc of the 250-mile network is above ground.