A French law that bans supermarkets from destroying unsold food and obliges them to give it to charity is irritating retailers who say they already make a big effort to fight waste.

Under the law, stores of more than 400 sq m have until July 2016 to sign contracts with charities or food banks, and to start giving them unsold produce.

It follows a media campaign run by a young right-wing politician, Arash Derambarsh, who says he was outraged by the sight of homeless people last winter scrambling in supermarket bins.

A local councillor in the Paris suburb of Courbevoie, Derambarsh began his campaign by collecting the unsold food and handing it out to the needy. He then launched an online petition, which helped create momentum for the new law.

While broadly welcoming the law, charities are also wary about ending up with more food than they can handle.

Jacques Bailet, president of the French Federation of Food Banks (FFBA), also says there is a risk charities will not be able to cope.

"Our food banks are going to need more staff, more lorries, more refrigerated rooms. But to get all that, we will need money - and money is pretty scarce these days," he says.

The FFBA already collects 35% of its donations from supermarkets, and Bailet praises retailers for their co-operation.

The supermarkets themselves, meanwhile, feel aggrieved at being portrayed as food wasters.

"It is wrong to point the finger at the big supermarkets when we represent just 5% of food waste in France. In fact we are exemplary - with 4,500 stores having already signed contracts with associations," says Jacques Creyssel of the Federation for Commerce and Retail.

Others point out that of the 7.1m tonnes of food wasted in France each year, according to the Ministry of Ecology, 67% is wasted by consumers themselves, and another 15% by restaurants, while shops and distributors waste 11% of the total.