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Commuters’ mobile phones could be monitored to reduce overcrowding in London's Tube stations and increase revenue from advertising.

During a four week trial before Christmas, Transport for London (TfL) followed 5.6 million phones via Wi-fi, identifying what passengers were doing at particular points of their journeys – such as entering or exiting a station, changing between lines or passing through a station while on a train.

TfL said the data had helped it gain a greater understanding of how people move through stations and how crowds develop, which in turn could allow it to put in place measures to improve customer experience.

The system could be also used to sell advertising, with prices dependant on the length of time travellers spend on platforms.

The study revealed that passengers use at least 18 different routes while travelling between King’s Cross St Pancras and Waterloo, and 40 per cent of customers don’t take one of the two fastest routes.

The Wi-Fi data, collected at 54 London Underground stations within Zones 1-4, was “de-personalised”, with nothing to identify individuals and no browsing data was collected.

No data collected through the trial was made available to any third parties while passengers could opt out.

Previously TfL has only been able to monitor where passengers enter and exit the underground, with no information on how people make their journeys between those points.

Anonymised phone data is seen as a far more accurate way to track journeys, and TfL is now assessing how to develop the monitoring system.

And it admitted yesterday that it could be used to track passenger movements in “real time”, according to The Times, with privacy campaigners expressing concern over the technology.

Renate Samson, chief executive of Big Brother Watch, told the paper: “Analysing movements of people via their device may provide unique analytical benefits but is still a process of tracking and monitoring as they go about their daily business.

"It is critical that the public are completely clear on what is being done, when, how and why, and how they can opt out.”

Val Shawcross, deputy mayor for transport, said: “We’re determined to use the latest technology to improve the experience of every passenger using transport in London, and I’m delighted the trial has been a success.

“The analysis of secure, de-personalised Wi-Fi data could enable us to map the journey patterns of millions of passengers and understand in much greater detail how people move around our transport network.

“It will provide real benefits helping TfL tackle overcrowding, provide more information for passengers about their best journey route, and help us prioritise new investment where it’s most needed.”