Scanners are to be placed outside stores from Pret a Manger to Aldi to track people through their smartphones’ wifi signals.

One thousand of the sensors will be used to measure the numbers passing or entering, known as footfall.

The idea is that the information could help revive dying high streets threatened by the rise of internet shopping.

Pizza Hut, Aldi and Pret A Manger are among the retailers that have signed up to have the scanners installed instore

It could lead to changes in bus timetables to make shopping visits easier, or identify times of the day when free parking would help retailers. In theory, it could even be used to decide that some town centres are beyond saving.

But the idea that shoppers will be tracked through their phones’ wifi signals is controversial. Many balk at the rise of the surveillance society through CCTV cameras, automatic number plate recognition and smartphones.

The campaigning group Big Brother Watch warned that many people do not realise they are being scanned in this way and that such surveillance systems are open to abuse.

The team running the project, however, insists all the information will be made anonymous, removing any possibility of the data being linked to a specific individual. And shoppers can opt out by choosing to turn off wifi signals on their phones.

The SmartStreetSensor Project is funded by the Government’s Economic and Social Research Council.

The retailers so far signed up to take part include Pret a Manger, Aldi, Oxfam, Pizza Hut, Superdrug, Thorntons, Dixons Carphone, Patisserie Valerie, Jack Wills, Tortilla, The Entertainer, Eurochange, Itsu, and Ed’s Easy Diner.

The 1,000 sensors that will be placed in 81 towns and cities around the country have been developed in a partnership between the Local Data Company (LDC), which tracks the health of high streets, and University College London.

Data from LDC shows that major chains closed 1,043 high street stores in 2015, while independent traders opened 593.

One thousand of the sensors will be used to measure the numbers passing or entering by detecting their smartphones' wifi signal

LDC said the scanners will help to answer questions about which stores get the most visitors, identify which towns are losing most shoppers, and show whether the arrival of coffee shop chains provides a boost.

The retailers involved in the study will be able to see the flow of shoppers to particular locations and could use the information to identify busy times when stores can try different ways to get people through the door, for example by using window promotions.

Renate Samson, chief executive of Big Brother Watch, said these kinds of scanning systems are open to abuse.

‘None of us really understand what our phones are connecting to when we are in the street, on the Tube or in shopping centres,’ she added.

‘There have been plenty of studies to show these open wifi connections on smartphones mean that you can almost plot an individual’s movements right back to their home.

‘It is a real concern that people will not know they are being scanned through the wifi on their phones. If they don’t know that it is happening, how can they choose to opt out by turning it off?

‘It would be a good idea to send people a message on their phone to let them know that this is going on and telling them that they can opt out by turning off the wifi.’