Smart bins in London are monitoring people's smartphones to help companies target their advertising.

Renew, the company behind the recycling bins, installed 100 of its recycling bins, primarily in the City of London, before the 2012 Olympics. Twelve of those bins were fitted with digital tracking devices.

The bin's digital screens show useful information like live Tube updates and breaking news, in addition to digital adverts.

However, Renew recently fitted several of the sophisticated bins with phone tracking technology as part of an effort to bring internet tracking cookies to the real world.

The 12 bins with the technology record a unique identification number, known as a MAC address, for mobile devices in the vicinity that have Wi-Fi switched on.

This enables the Renew bins to monitor data including the "movement, type, direction, and speed of unique devices".

The stalking is currently taking place as part of a trial project demonstrating the potential of targeted personal advertising.

"It provides an unparalleled insight into the past behaviour of unique devices – entry/exit points, dwell times, places of work, places of interest, and affinity to other devices – and should provide a compelling reach data base for predictive analytics (likely places to eat, drink, personal habits etc.)," reads a blog post on the start-up's site.

In tests occurring between 21-24 May and 2-9 June, over four million events were observed, with over 530,000 unique devices monitored.

The bins are able to track passersby by using technology developed by Presence Orb, which was described as a“cookie for the real world”when it was launched by owners Presence Aware in March this year. This was in reference to the cookies that monitor web browser's movements as they surf online.

People can opt out by visiting the Presence Orb website which details information on how companies can avoid having the MAC number on their mobile device monitored by their technology.

In the European Union,websites are legally required to notify users if a tracking cookie is placed on their computer, but Renew maintains that its cookie monitoring, which doesn't alert people, is not infringing on anyone's privacy.

Renew London CEO Kaveh Memari told Quartz: "London is the most heavily surveillanced city in the world… As long as we don't add a name and home address, it's legal."

Renew is looking to expand the technology to all of its bins across London in addition to those that it has in New York, Dubai and Kuala Lumpur, according to Quartz.