Transport for London (TfL) has turned on its WiFi tracking. 

London Underground passengers will be tracked through the wi-fi beacons on their smartphones to help improve network management and potentially yield commercial opportunities.

The plans have been in preparation for years following a trial of the system in 2016.

TfL said the tracking data - collected as smartphones pass wi-fi transmitters - would help it understand how people travel and would help send targeted information about avoiding congestion, although it could also yield commercial benefits.

The transport authority has had funding issues since it lost a £700m annual operating grant from central government and the data could allow it to price advertising based on where it sees the heaviest footfall.

Lauren Sagar, TfL’s chief data officer, said: 'Transparency, privacy and ethics need to be at the forefront of data work in society and we recognise the trust that our customers place in us.'

Technology experts have raised concerns about whether the data can be properly anonymised or kept confidential.

TfL said it would put up signs telling passengers that they can opt out of having their data collected by turning off their wi-fi - although this would also mean the loss of internet connection. 

Wi-fi is now available at over 260 Tube and 79 London Overground stations across the TfL network.