A "fake fingernail" that sends sunbathers a mobile phone alert if they are at risk of getting sunburnt has been unveiled at the world’s biggest gadget show.

L’Oreal, the French beauty giant, has unveiled a tiny ￡30 sensor designed to be placed on a thumbnail. It detects ultra violet rays in order to tell when wearers have spent too much time in sunlight and their skin is at risk.

The UV Sense device was unveiled at the Consumer Electronics Show, a mass gathering of the technology industry in Las Vegas attended by 200,000 people, that is meant to showcase the world’s key innovations.

L’Oreal said the release of a previous gadget, a patch that users wore on their skin, led to 34 per cent of people applying sunscreen more often, and 37 per cent going into the shade more frequently.

It is 2mm thick and is attached to the thumbnail, an area that receives the optimal amount of sunlight to detect if the wearer is at risk.  Inside it has a collection of sensors and silicon connectors, but runs without a battery, connecting to a smartphone through a contactless chip similar to those found on credit cards.

The smartphone app can store months of data to show wearers how much they have been exposed to the sun over time. L’Oreal has designed the device with researchers at Northwestern university in Illinois.

It says it has sold more than a million of its previous device, a skin patch released two years that detects sun rays. It plans to put it on sale in the UK in 2019, and is aiming for it to cost around $40 (￡30).