Tesco has installed 4ft smoke devices in some of its stores to "deter thieves" from raiding shops following a spate of break-ins.

The devices are reportedly wheeled onto the shop floor after London stores at high-risk of burglary have closed.

If motion sensors are activated, dense smoke funnels out and fills the room to make it difficult for items to be taken.

The machines are also fitted with a CCTV camera.

A message on the front of the device reads: "Warning: You’re being watched. Smoke screen security fog in operation."

Tesco has stressed that the units are not new and are not expected to be expanded to every store.

Photos of the machines have been shared online, however a unit spotted in a central London shop was not in use and has since been removed.

"It’s a device that will stop robbers quite quickly. The smoke isn’t harmful, but it will stop you seeing what’s in front of you, which makes taking anything very difficult," a source told The Sun.

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"Once the machine goes off they’re also on camera. The box shouldn’t be left out for customers to see, but overnight it would be put out in the store."

Some 400 branches - including petrol stations - have been fitted with smokescreen devices.

It comes as shoplifting and attacks on shop workers have surged across Britain.

The latest figures - released in February - show that 1,300 incidents of violence and abuse take place a day.

Incidents of violence and abuse in shopping centres had risen by 65 per cent from 2021-22 to 2022-23, according to a report by British Retail Consortium.

Body armour suppliers said the demand from shopping centre security staff for stab vests has soared over the last two years.

Tesco has been approached for a comment.