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Minister Christophe Castaner said the units should be used more as an alternative to a ban and to cut accidents, while helping drink-drivers who face losing work if their licences are removed.

Prefects can decide whether to use the system but only 55 departments do so at present.

The technology is mandatory in school buses but car units are expensive, costing €1,700 to fit, though they can be rented for €100 a month.

The legal limit in France is 0.5g/l, or about two units – two small beers or glasses of wine – or 0.2g/l for new drivers on probationary licences.

The offence rises from an ordinary fine and points off the licence to the more serious »åé±ô¾±³Ù category from 0.8g/l.

The use of the new equipment as an alternative to a ban is allowed at the 0.8-1.8g/l level.