But the giant McDonald's fast food empire has now launched a mission to produce an eco-friendlier French fry.

A group of socially conscious investors has sent the multinational on a hunt to find a new type of potato to replace its best-used spud, the Russet Burbank, which it uses as its mainstay in both the US and the UK.

The company still relies on the 130-year-old variety for many of its fries, even though the potato takes a long time to mature, gulps water and falls victim to rot and other diseases which demand high pesticide usage. Worse, worms in England have developed a taste for the Russet Burbank, forcing farmers to use more chemicals.

Investors want McDonald's to help cut pesticides to protect the environment and farm workers' health.

The firm's potato hunt is just one of a series of projects launched to green its image. The giant, stung by scathing criticism of its environmental record, has already switched its UK delivery fleet to burn used vegetable oil from its deep-fat fryers rather than petrol.

Mitch Smith, McDonald's agricultural products director, said: "If we can find a variety that requires less inputs, that's something we're looking for. To date, there are not a lot of varieties that perform consistently enough."

Allan French, a globe-trotting specialist who oversees potato varieties that feed a sprawling fry-processing empire stretching from the US to China, says finding a replacement has been elusive. "We're looking for the silver bullet to replace the Russet Burbank," French said.

The quest could have a huge impact on farmers and consumers, whether they eat at the chain or not. McDonald's serves up two million meals a day in Britain alone, most with at least a portion of its skinny chips.

It buys 324 million lbs of potatoes in Britain alone every year. In America, the figure soars to 3.4 billion lbs.

The sheer scale of McDonald's consumption gives the company the power to dictate whether a variety is a success or winds up in the less-lucrative oven chip section of the supermarket freezer section.

In America, what spud you get depends on the time of year. In August, September and October, the fries are Canadian. By November, Ranger Russet fries hit the fast-food joints. Better-storing Umatilla Russets – the last US potato variety approved by McDonald's back in 1999 – fill the bellies of consumers from late December until the end of February.

Potatoes aren't just judged on their taste, texture and golden-brown appearance. Varieties are also selected for their yield, hardiness and ability to resist disease. Unfortunately, the Russet Burbank does not offer the best balance.

Finlay Dale, government scientist with the Scottish Crop Research Institute (SCRI) near Dundee, and an expert on potato selection, explains: "They're quite susceptible to late blight and there's been a change in recent years that has knocked out a lot of the Russet Burbank's resistance.

"There are no quick fixes in trying to find a replacement for this. McDonald's are looking for traits that will improve their varieties and reduce their reliance on agrochemicals.

"Something like 65 per cent of the potato land in England has a nematode (worm] problem, which requires some application of sprays, otherwise you get vastly reduced yields.

"If you're spending six or seven years of trialling, then three years with testing, you can see it's not a case of 'we need another variety next year'.

"It will be ten years before the varieties I'm working hit the market."

Environmentalists remain sceptical about McDonald's green credentials. Patrick Harvie, the Green MSP, said: "There are structural reasons why global big food corporations like McDonald's are always going to find it hard to be green.

"Small local businesses which buy locally grown potatoes are always going to be more sustainable. There is no need for produce like potatoes to be shipped around the world to be eaten.

"This is a company that has a long way to go before it wins environmental credibility and convinces the likes of me. It has been responsible for some appalling environmental damage in global terms, from littering in our streets to deforestation in South America."

The SCRI's Dale believes the future of potato cultivation, especially when it comes to meeting the stringent demands of corporations like McDonald's, lies in genetic modification technology.

"There is the GM route," he said. "Instead of mixing all the genes up and hybridising, we can say, 'Well, we want to improve just one trait, the rest are perfectly good'. It would be a very attractive proposition, but I don't think the public accept it at the moment."

He added that the rising cost of fuel will have a knock-on effect of pushing up the price of many pesticides, which may force producers to answer "some big questions" in the next 20 years: "I do understand the concerns. But we have increasing population and we have to feed it somehow."