Transport for London is conducting a contactless payments trial that could allow passengers to pay for journeys using a smartphone in future.

TfL is already in the process of upgrading barriers on Tube and rail networks to allow payments using debit cards embedded with near-field communication (NFC) technology. The service is expected to be made available to the public by the end of the end of the year.

According to TfL, the upgrade also means that smartphones equipped with NFC systems could also be swiped against payment sensors to pay for journeys, and is currently overseeing a trial to see how the technology works.

"The upgrade to our readers to accept contactless payment cards also makes them capable of accepting suitable payment applications on mobile phones," said TfL director of customer experience Shashi Verma.

"In principle, mobile phones with a Visa, Mastercard or AMEX payment application could be accepted on our services. At this stage, mobile phones with pre-paid cards will not be accepted.

"We are testing to see how the devices perform on the system and welcome any innovations which improve the services and choices we are able to offer customers."

However, SkyNews reported that the technology is currently deemed 'too slow', and will not be rolled out until transaction speeds are increased.

Contactless payment card payments have been allowed on London's buses since December 2012, with more than 11.5 million journeys made using the payment option.

However, although contactless payments technology has become more widely adopted, with Visa claiming UK customers spent £460 million using this method last year, there has been slow uptake of NFC smartphone payments so far.

A joint venture between Vodafone, EE and O2, named Weve, aims to create mainstream demand for smartphone payments at high street retailers, with the roll out of its service planned for next year.

Others have attempted to popularise the technology, such as Visa with its PayWave mobile payments platform.