Air-conditioned shops in France will be ordered to keep their doors closed or risk being fined, a minister said Sunday announcing an upcoming rule to combat energy wastage.

Leaving the doors open when the air conditioning is on leads to "20 percent more consumption and... it's absurd," French Minister of Ecological Transition Agnes Pannier-Runacher told RMC radio.

The minister said there were also plans to restrict the use of illuminated signs.

"In the coming days, I will issue two decrees: the first will widen the ban on illuminated advertising, whatever the size of the city, between 1 am and 6 am", with the exception of airports and stations, Pannier-Runacher told the Journal du Dimanche newspaper.

"The second will ban shops from having their doors open while the air conditioning and heating are working".

Some cities in France -- which like other parts of Europe has been gripped by a heatwave recently -- passed municipal by-laws in July, imposing fines for offending air-conditioned shops.

The government now plans to extend this to the whole country, with a fine of up to 750 euros ($766) -- but will emphasise the education of shopkeepers in the first instance.