O2 tracks and sells what customers are doing on the Tube

The data is anonymised and aggregated, however.

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If you're with O2 and regularly use the London Underground, you should know that the network is trying to track your phone and pass that information on to advertisers. If that sounds like a huge invasion of privacy, well, in some ways it is -- however, the silver lining is that all of the data is anonymised and aggregated before it's visible to any marketing folk. O2's owner Telefonica struck the deal with Exterion Media last summer, the latter of which owns every billboard on the subterranean transport system. It then came into effect on Thursday, monitoring customers' journeys, app usage and browsing activity over Wi-Fi.

The idea is pretty simple. By gathering all of this information, Exterion Media can show advertisers the types of people that make particular journeys. Knowing that large swathes of fashion-conscious Instagram lovers stand on the platform near Covent Garden, for instance, could inspire ASOS or Urban Outfitters to take out an ad. Similarly, knowing when certain users descend on a station -- Stratford is normally a place for shoppers, but the Olympic Park occasionally attracts sports fans -- could help companies figure out when to debut a highly targeted poster.

O2 has 25 million customers and expects to track roughly one million journeys each day. If you're worried about your privacy, there's an obvious solution -- switch off your phone's Wi-Fi connection and do something else while you're sitting on the Tube. Sure, you'll have to wait a little longer to check Twitter or Facebook, but there's always an alternative, like listening to some podcasts or ploughing through your Pocket queue.