Coca-Cola is testing a vending machine that automatically raises the price of the world's favourite soft drink when the temperature rises.

The US soft drinks group has been investing heavily in vending machines to put its products "within an arm's reach of desire" in schools, workplaces, garages, leisure outlets and public places such as stations.

And it has been testing hybrid machines that combine drinks dispensers with petrol pumps in garages and with stamp machines in post offices.

Other developments include machines that take credit cards and one which monitors sales, orders products running low and can call a central point if the machine goes wrong.

But the new machine, believed to have been tested in Japan, may well create controversy by using hot weather to charge extra. One rival said the idea of charging more when temperatures rose was "incredible".

Most soft drinks companies were trying to make their products more accessible, he added.

Details of the new machine emerged in an interview given by Doug Ivester, Coke's chairman, to Veja, a Brazilian magazine.

Mr Ivester said it was designed to reconcile supply and demand by raising the price when demand increased.

"Coca-Cola is a product whose utility varies from moment to moment," he said. "In a final summer championship when people meet in a stadium to enjoy themselves, the utility of a chilled Coca-Cola is very high. So it is fair it should be more expensive. The machine will simply make this process automatic."

The Atlanta-based company said none of the new machines were currently in use but that it was evaluating the technology as it did with every other vending machine innovation. "We have no plan to put them in the marketplace," it added.

Coca-Cola shares have dropped sharply this year after four consecutive falls in quarterly profits. It has been hit by economic turmoil in emerging markets, a health scare in Europe and aggressive competition from main rival PepsiCo.