A few minutes before noon this morning, a teenage boy or girl on one of the All England Lawn Tennis Club's immaculate outer courts will peel open the first tin of balls for this year's championships, unleashing its signature thwip. Over the next fortnight another 48,000 will be carefully tossed to the tournament's 560 players. But while the eyes of the players - and the thousands of us watching from afar - will struggle to follow their every move, few will give more than a passing thought to the furry greenish yellow objects hurtling to and fro at speeds of up to 130mph.

Around the world a handful of people may be watching a little more closely though. The staff at Dunlop Slazenger, perhaps, celebrating its centenary this year as official suppliers of balls to the Wimbledon championships. The women from the factory in Barnsley who have made the balls every year since the second world war. Maybe even the workers in Bataan, in the Philippines, who will take over the job before next year's tournament. For them, the balls represent something different - the end result of hours of repetitive hard work, the final act in a meticulously choreographed ballet played out across four continents, and, above all, a livelihood.

Although a special batch is produced for the tournament, you can buy the Wimbledon Ball in just about any sports shop in Britain. JJB Sports on the Euro Retail Park in Ipswich, for instance. A big hangar of a place, crammed with tracksuits, sports shoes, rackets, football kit. And balls. A tin of four Wimbledon tennis balls costs £7.99. There is a big gold rosette at the top of the metallic blue tin: "Official Ball to Wimbledon. Slazenger 1902-2002. 100 Years." Then underneath: "Wimbledon Ultra Vis, with Hydroguard Water Resistant Cloth. International Tennis Federation Approved."

You might think that there isn't much to a tennis ball. Just a lump of hollow rubber with some cloth stuck on it. But you would be wrong. At the Dunlop Slazenger factory in Bataan there is a whole laboratory dedicated to the development and testing of tennis balls. There are people in the company who spend their entire working lives thinking "blue sky" thoughts about the future of tennis balls. There are academics in the engineering department at the University of Loughborough who spend their days pondering the subject. There are professional tennis managers in Cheltenham paid to test sample balls on court.

The tennis ball has come a long way since Slazenger provided its first hand-sewn, wool-coated balls to Wimbledon in 1902. Every ball is identical now. It is measured for its bounce, its size, its internal pressure. Its wool must be of a good quality, each fibre around three inches long. The tin in which the ball is packed must be pressurised to keep it from going soft.

Tennis balls are big business. They make up about a fifth of the £165m annual turnover of Dunlop Slazenger, which in turn makes about a fifth of the 240 million tennis balls made each year worldwide. And though its is the only name on the tin, Dunlop Slazenger is just one of dozens of companies that share a stake in the balls rolling from tins in Wimbledon today. All over the world right now, there are ships chugging across oceans towards Manila with a breathtakingly diverse range of ingredients. Clay from South Carolina, sulphur from Korea, silica from mines in Greece, magnesium carbonate from Japan and zinc oxide from Thailand are but a few of the substances used to vulcanise the rubber and give it the right amount of stretch and bounce. The tins come from Indonesia; some of the dyes are shipped from the UK.

To reach Dunlop Slazenger's Bataan factory, you must take a ferry from Manila to the little port at Orion, then drive for 40 minutes or so through lush forests and past tumbledown houses to the export zone at Mariveles. Along the way, Christian slogans brighten the journey: "Commit your Work to the Lord and He will Crown your Effort With Success," cries one roadside signpost. Down, then, into a valley where the tropical greenery seems to be winning a battle for space with the factories sheltering at the bottom.

There's a ramshackle air to the Bataan economic zone, set up nearly three decades ago by the Philippine government in the hope that western companies would flock in. If ever there were glory days, they are gone. A huge cinema stands empty; factories abandoned as companies join the search for even cheaper labour in China. Outside the Dunlop Slazenger plant a line of people wait for interviews, hoping to bag one of the jobs transferred here from Barnsley.

Inside, the factory is a calm, neat place with fresh blue and white paint and a display case showing the stages of ball manufacture. There is a canteen where management eat with staff, and the offices have a view to the floor. The windows cut out most of the noise. Here we cut open one of the balls from JJB Sports to find a yellow stripe inside. That means it was made in February 2002.

Open the door, though, and the machines take over. Neck-bendingly high, gargantuan machines whose background roar is overlaid by endearing whoops, pings, booms and gunshot sounds as they grind through their work. The workers' white polo shirts and blue jeans are just specks in this Brobdingnagian landscape.

High on a platform in the midst of all this activity, Ricardo Caranto feeds the monster. Every few minutes, to the sound of a hoot, bleep or buzz, he opens a hatch and throws a chunk of rubber and a consignment of clay filler or chemicals down a chute into a huge mixer below. Throat-catching dust flies up in his face from the churning machine, but when a manager asks why he isn't wearing his mask, he says that the factory's dust extraction is pretty good.

Caranto works shifts, six till two, two till 10, then a 12-hour night shift, and he has been here two years. The work isn't too bad, he says - each week he is moved to a different task so that there is variety. He used to work in a Korean shoe factory but the Dunlop Slazenger plant pays better money: 395 pesos a day - about 65p an hour for the job he does. Like most of the employees here, he is not local. He came here five years ago from Central Luzon, the Philippines' main rice-growing region, to find work. He rents a four-roomed house with his wife and two children half a mile from the factory, and pays 850 pesos (£11) a month in rent.

Down below him, the machine emits huge blankets of Plasticine-like pink rubber, which pop, bubble-wrap style, as they are fed through metal rollers. Along the way they are installing another of these great beasts, shipped here from Barnsley.

Cherryle Camara works in the forming room where the balls are finished with two interlocking, elongated ovals of cloth. Several times in a minute she slots her two strips of glued material into a clamp, puts the ball in, pulls the clamp over the rubber and removes the near-finished product. It is one of the most difficult jobs - and if there's a pucker in the cloth, the ball will be rejected.

Camara has been here a year and a half, and she is hoping to stay. Her mother works in a shoe factory, her sister at a clothing firm. Her father is unemployed, though, since being laid off by a Korean garment firm. The family share a two-roomed house in one of the breezeblock-grim government accommodation areas just up the hill, and they pay £20 a month for it. Camara earns about £5 a day.

Job insecurity is a constant theme here. The workers have become accustomed to factory closures, and if you ask them whether they plan to stay, they will reply: "God Willing," or "That depends on the company." Even working for a firm with 27 years of history here, they fear their destinies are not entirely their own.

And here, at the end of the line, is a tin of balls just like ours, trundling along with countless others ready to be packed. Its core is four-tenths rubber, six-tenths filler and chemicals. Its outer coating, we are told, is cloth made of a wool and synthetic mix. But what rubber? What cloth? And from where?

Two kinds of rubber, actually. The rubber in the core of the ball comes from Malaysia, where it is delivered to a processing plant in Prai, near Penang, from small plantations in the area. Then there's the rubber which is mixed here with petroleum naphthalene - the air is thick with the solvent in some parts of the factory, despite the ventilation - to make glue for the balls. This is from Basilan, in the southern Philippines.

In the public imagination, Basilan is a wild west place, crawling with bandits and kidnappers. It has a home-grown Muslim fundamentalist terror group, Abu Sayyaf, which has been linked to Osama bin Laden's al-Qaida. Recently, there was a fierce shoot-out between the group and US special forces sent to the island to sort out the trouble.

Even the presence of armed guards and a US tank by the port can hardly prick the outwardly tranquil air, though. Tiny wooden stilted houses fringe white, sand beaches and tropical forests surround the island's main port of Isabela.

Here in the processing plant on the Latuan plantation, gangs of labourers sluice minced, coagulated rubber with water before baking it into bales. The unprocessed "cup lumps" look and smell like rancid mozzarella, and the air is heavy with it. Antonio Jayson works here for 145 pesos a day, around £1.90. He's a newcomer, a refugee. Jayson's family had to flee five months ago from their home in Tumahubong, up in the mountains in the centre of the island, after rebels threatened to kidnap his teacher wife, Edwina.

"This is just OK," he says. "It's a job. I will stay here because I'm afraid to go home, and because I prefer it here. But I don't want my children to work on a rubber plantation. I would like my son to be educated, to have a good job. Maybe he can become a teacher."

In truth, terrorism is not uppermost in the minds of most residents of the Latuan plantation, even though they need full-time security. They are perhaps more worried about whether their rubber business can survive without investment. Seven years ago workers here were given ownership of the land under an agrarian reform programme. But the withdrawal of the big plantation companies brought its own problems. Sections of the estate are now being planted with bananas, which bring cash returns faster than rubber trees, and in some areas rubber trees well past their productive life stand unfelled.

Gener Martalla, 39, grew up here and every morning he rises at six with his hockey-stick shaped knife to begin tapping, cutting thin strips of bark from the trees so that the white latex will drip out into a waiting cup. Under a tall, gothic arch made by two rows of trees, he takes a break and sits on an upturned bucket to talk.

"The hours were longer before," he says, "but we still work seven days a week. And we have no money. It's very hard to plant more rubber because it is seven years before you can cut it. It's very hard for us. But I think there will still be rubber here in 20 years."

Martalla's wife, Ramanita, helps keep the pigs that run between the neat wooden bungalows which house the 500 families here, with their flower gardens and vegetables. But the whole Martalla family, including the couple's 19-year-old daughter, must live on the 175 pesos (£2.30) a day that Gener earns.

Life may be hard in Basilan, but the rubber workers in Malaysia, where the rest of our rubber is grown, would love to be in Martalla's shoes. There, workers on the larger plantations do not own the trees they tap and in the past few years there has been increasing tension over pay. These employees, who earn around £50 a month in UK money, have been demanding a minimum wage to replace the current system of pay which is based on how much they can collect, estate yields and commodity prices. They want a minimum of around £130 a month. The poverty line in Malaysia is set at around £80 a month.

So much for the rubber. The Philippine plantation gets around 25p a kilo for it. Each ball contains roughly 25 grammes of rubber, which at that price would make about 0.6 pence per ball. But what about the part of the ball that we can see - the greenish yellow cloth covering? It's made in the UK, in Dursley, Gloucestershire. The wool, from cross-bred sheep, travels 11,800 miles to get there from New Zealand before being shipped 6,700 miles back to Asia to be made into tennis balls.

Our sheep, according to Fred Heap, who buys wool for Milliken, the American conglomerate which owns the firm that makes our cloth, came from New Zealand's South Island, maybe South Canterbury or Otago.

The length of the wool and the quality of the cloth matters. Christine Sanger, who was laid off last month after 26 years at the Barnsley factory, still remembers the time John McEnroe held up a too-furry ball to the television camera and remarked that it needed a haircut. There would have been trouble about that, she says.

Her pride in the work she has spent her life on is palpable. "Barnsley always produced the best balls," she says. "We produced for the best tournaments in the world, and Wimbledon was the jewel in the crown. When I started, those balls were virtually hand-made, and only the best materials would do."

The factory was a happy place in the old days, too. "It was smashing when I first worked there. It was speed work - you had to be quick, but we used to have a laugh. It was a factory where you got your family in. You worked there, you got your daughter in, your cousin. It was a family factory."

Even as the fears for the future grew, the Barnsley workers still believed they would survive. The local authority had applied for European funding to help build a new, hi-tech plant at Grimethorpe, nearby.

"We were going to be the crème de la crème. There would have been fewer jobs, but they would have been safe," Sanger says.

But it was not to be. Before Christmas last year, during a temporary lay-off, staff were called back to be told that the factory would close when the Wimbledon tournament order was complete. The atmosphere became increasingly bitter. Workers at the factory even threatened to strike when they heard that staff from Bataan were coming over to see how things were done.

Now Sanger, who, as a union rep, fought tooth and nail against the casualisation of labour, has been forced to take on agency work. At 56, she is doing nights for £5 an hour in a warehouse owned by Morrisons' supermarket. Her redundancy amounted to just £11,000, and she feels angry and disillusioned.

"My family's always been Labour," she says. "My husband was a miner. I've picketed, I've been on strike. What have I been doing all these years? Our MP didn't say a thing when they closed the factory; never even raised it. These tennis balls are part of our English heritage, and the only person who said anything was the Conservative leader Iain Duncan Smith."

In the end, the closure of Barnsley was a quiet affair. The redbrick former munitions factory closed its doors a couple of weeks ahead of schedule in the middle of last month after the tournament balls were ready. The 134 workers came back at the end of May to collect their final pay, and that was that.

For those of them who watch the last batch of Barnsley-produced balls taking centre stage at Wimbledon today, there won't be the same thrill of anticipation, of excitement at seeing their work in action, that they felt in previous years. For them, the game is already over.