How do US-style 'alcohol bracelets' work?

People who commit alcohol-related crime will be made to wear ankle tags that monitor their drinking under a new scheme to be trialled in four London boroughs.

But how do they work?

The "sobriety tags" will be worn by offenders 24 hours a day.

They will measure a person's sweat and air emissions from the skin every 30 minutes to see if it contains alcohol.

The device appears to be tamper-proof with multiple sensors ready to detect obstructions and removals, according to one manufacturer.

What happens if a positive trace is found?

An alert will be sent to the offender's probation officer.

The person can then be recalled to court, where they could be fined or be re-sentenced.

Has the concept been used before?

A similar scheme was successful in the US.

Actress Lindsay Lohan was forced to wear the bracelet after breaching her bail conditions for her drink-driving arrest.

Former White House Drugs advisor Professor Keith Humphreys says the idea has had a "transformative effect" in the US.

Mayor of London Boris Johnson hopes the success of the scheme can be replicated to tackle "alcohol-fuelled criminal behaviour" in the capital.

If the scheme is effective it could be rolled out across the UK.