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Pavement adverts have been temporarily legalised in three French cities as part of an 18-month experiment into their impact.

Since Christmas Day, advertisers have been allowed to paint temporary biodegradable advertisements on pavements in Bordeaux, Lyon, and Nantes, after a decree was published in the Journal Officiel.

These adverts must be applied via a stencil using water or chalk-based biodegradable paints, the decree said. They must wash away naturally or be removed after 10 days, cannot exceed 2.50m2 and must be at least 80m apart.

The experiment in the three cities will be re-evaluated every six months during the 18-month period - and authorities reserve the right to terminate the study at any time. A key part of the study will examine whether there is an increase in accidents or falls at advertising sites, though the rules also state that the paints used must be non-slip.

"Guerrilla advertising" techniques like this have been seen in French cities for some time, but marketers who use them risk punishment as they break the law. In 2015, authorities in Paris were forced to clamp down on such advertising by stepping up enforcement.

A spokesman for the Résistance à l'agression publicité organisation criticised the 'additional pollution' of pavement adverts. Khaled Gaiji told Francetvinfo: "The method may be clean, but its purpose is dirty … Our mental and actual environment is being invaded by these advertisements."