Nigeria has one of the highest deforestation rates in the world and conservationists say the charcoal industry is fuelling the problem, as demand for charcoal grows internationally.

Last year the country imported more than 10,000 tonnes of charcoal from Nigeria, according to UN figures.

"Deforestation in the country is alarming and disheartening for some of us who work in the conservation sector," says Stephen Aina, a conservation officer with the Nigerian Conservation Foundation.

He says forests in Oyo State and in areas sharing a boundary with Benin have been largely destroyed, because of their location close to Lagos port. Kwara State, he says, is now one of the major hotspots of charcoal production in the country.

Poland, a major exporter of charcoal, also imports the fuel from Nigeria before re-exporting it, according to a World Wildlife Fund Germany.

It's a problem in other African countries too. “Tree cover loss in the Democratic Republic of Congo reached a record high in 2017, increasing 6 percent from 2016,” a recent report by the Global Forest Watch said, saying agriculture, artisanal logging and charcoal production were to blame.

It isn't illegal to source charcoal from tropical forests, but most British retailers do claim their supply chains originate in sustainable woodlands.

The majority of the bags, though, have no information about the country of origin, let alone specific forests. But most do carry the logo of the Forest Stewardship Council (FSC) - the world's biggest certification scheme of wood products.

"I am sure there has been tropical charcoal coming into the UK market,” admits FSC boss Kim Carstensen. “I am sure most of these would have been without the FSC logo," she adds, "but I cannot guarantee that there hasn't been any problem with some of the FSC certified materials.”