It hardly shrieks of billion-dollar glamour. The US nerve centre of the world’s largest retailer, Wal-Mart, consists of a collection of low-slung prefabricated buildings along a four-lane highway in north-western Arkansas. Wal-Mart’s head office is hundreds of miles from the nearest big city. It isn’t even handy for the state capital, Little Rock, which is three and half hours’ drive away.

But hopeful merchants beat a path from all corners of the world to hawk their wares here, in a series of bare Perspex rooms along a “supplier corridor”. Staff work in spartan cubicles and reminders of the retailer’s low-cost culture are constant – in an employee lounge an honesty box invites payment for tea and coffee with a blunt message: “Drinks are not free.”

It was nearby, in the main square of the modest town of Bentonville, that Wal-Mart’s founder, Sam Walton, opened a discount store, Walton’s Five and Dime, in 1951. That shop, now a museum, helped spawn a retail empire that spans 8,100 stores in 15 countries generating $401bn (£248bn) of revenue annually. With a market capitalisation of $210bn, Wal-Mart is worth as much as the gross domestic product of Nigeria.

Four of America’s 10 richest individuals are from Wal-Mart’s low-profile Walton family, which still owns a 40% controlling stake. The company’s portfolio ranges from superstores in the US to neighbourhood markets in Brazil, bodegas in Mexico, the Asda supermarket chain in Britain and Japan’s nationwide network of Seiyu shops. Wal-Mart gets many of its products from low-cost Chinese suppliers. The pressure group China Labour Watch estimates that if it were a country, Wal-Mart would rank as China’s seventh largest trading partner, just ahead of the UK, spending more than $18bn annually on Chinese goods.

Perhaps more than any other firm in America, Wal-Mart divides opinion. Unions loathe its relentless downward pressure on wages and its refusal to allow workers to organise. The company has been accused of unfair treatment of older, more expensive, employees. It is facing one of America’s largest class-action lawsuits alleging wage discrimination against women and its hypermarkets are routinely blamed for squeezing small shops out of business.

“This is a company with a record of exploitation,” says Jill Cashen, spokeswoman for a pan-union campaign group, Wake Up Wal-Mart. “They have not shared their wealth. When you spend your money at Wal-Mart, you’re contributing to the wealth of one very rich family and not very many other people.”