Britain-born Robinson has been a Hong Kong resident since 1985. She saw her first bear farm in 1993 and was horrified by the terror endured by the animals. She soon discovered that bear bile yields ursodeoxycholic acid, a valuable liquid used for treating high cholesterol, gallstones and liver disease.

'I knew I would be on a sticky wicket if people were dying and bear bile was their only remedy,' she says. 'I found out quickly that nobody will die for a lack of bear bile and there is an abundance of synthetics to take its place. I was so relieved. I felt that if I was disgusted by bear-bile farming the people of China would be disgusted, too.'

Since then, Robinson has been campaigning to end the practice. She has won support from influential people on the mainland and in Hong Kong, and, last weekend, China Bear Rescue was one of four animal-welfare groups featured at a fund-raising event in Beijing that was attended by a glittering array of celebrities and broadcast live on the internet.

'Using moon bears or any other kind of bear for bile extraction is a cruel and unnecessary practice,' says Harriet Tung, wife of shipping magnate Tung Chee-chen and sister-in-law of Hong Kong's first chief executive, Tung Chee-hwa. 'Moon bears are an endangered species whose numbers continue to decline.'

Tung says she can't understand how the government in Beijing can protect and venerate pandas 'while torturing and ignoring the welfare of the Asiatic moon bears'. Bear-bile farming remains legal on the mainland, albeit under tough restrictions, many of which are ignored by farmers. One ray of hope for the bears is that no new licences for bear farming have been issued since 1994. Despite that, officials estimate that 7,000 bears remain in bile-extraction units, although Robinson believes the number is closer to 10,000, when illegal operations are taken into account.

'Licensed farms are supposed to keep their animals in enclosures the size of a football field and only cage them during extraction,' Robinson says. 'Our undercover investigators have found that most farmers rarely let the bears out of cages. It's easier to take the bile when the bears are forced to rot behind bars.'

Following years of bile extraction, a bear's gall bladder becomes seriously diseased with gallstones, tumours and lesions, which frequently migrate to the liver and cause cancer if they are not removed.

'For the bear this is massively, massively painful,' says Heather Bacon, the veterinary director of Animals Asia, who is supervising the final stages of Monkey's operation and has participated in dozens of similar surgeries. 'The tumours and lesions cause chronic pain and there is usually a trickle of bile coming out of the fistula, which is very painful for the skin around the fistula site. Try to imagine having vinegar constantly poured onto an open wound.'

Last month, Bacon and Bando accompanied Robinson to Shandong to rescue Monkey and nine other bears. The farm's owner had agreed to hand over his licence and give up extracting bear bile after negotiations with the authorities and receiving a compensation package paid for by Animals Asia. As is the case in all of the charity's bear rescues, the Shandong farm was closed following close liaison with the government, a partnership Robinson has developed since 1993 and nurtured by presenting evidence that bear farming is unnecessary and cruel. One of her major breakthroughs has been to get support from doctors working with traditional medicine.

'Many products containing bear bile are not prescribed or recommended by traditional Chinese medicine doctors,' says Lixin Huang, president of the American College of Traditional Chinese Medicine. 'We ask bear farmers not to use the excuse of traditional medicine as a reason for farming bears because we do not need bear bile to save patient lives.'

'There are 54 herbal alternatives to bear bile,' says Robinson. 'The industry is being sustained by illegal exports. Just go down the main Chinese medicine street in Hanoi [Vietnam] to see bear-bile medicines which originate from bear farms in Shandong, Heilongjiang and Chengdu.'

When the Animals Asia team arrived at the Shandong farm, the farmer and his wife had removed the bears' metal jackets and ripped out their illegal catheters.

'The bears were in a dismal condition,' says Robinson. 'The farm was run down and we could smell faeces, dirt and infection. We found metal jackets and latex catheters full of bile and pus. One of the bears could not even lie on her abdomen because of the trauma from the wound the farmer had inflicted.'

All of the farm's 10 bears were large adults. One, whom Robinson named Oliver, had, by the farmer's own admission, been in a cage for 30 years, the upper limit of the average bear's lifespan.

Bears are towards the top end of the mammalian intelligence scale, with brain functions similar to those of dogs and dolphins. They can be taught and have an intellectual capacity that's on a par with a three-year-old child. For a creature with this intelligence to be crushed into a tiny space for three decades and subjected to daily torture through bile extraction suggests a degree of mental suffering that's terrifying to contemplate.

'You couldn't put any other animal [including humans] in this situation and hope to rehabilitate them,' says Bacon. 'The bears have an extraordinary ability to deal with excruciating pain and regenerate. But this is also a problem. The bears are so stoic that some people claim they are not in any pain. But that's completely untrue.'

Bacon and other scientists believe bears are able to survive the horrors of bile farming because they are natural hibernators with a unique physiology that can deal with being in a tiny space for a long time. But even bears have their limits.

'The Shandong bears were hostile towards us because, for them, the presence of any human means something hideous is going to happen to their bodies,' Robinson says. 'The vet teams knocked the bears out, did blood tests and prioritised them for surgery.

'We had been on the road back to Chengdu for two hours when we realised Oliver was in serious trouble.'

The 30-year-old bear had developed a high fever and the vet team suspected that bile had leaked into his abdominal cavity after the farmer yanked out his catheter, causing peritonitis, a condition that sends the victim into fatal toxic shock if not treated quickly.

'We were on the outskirts of Fenglingdu town, in Shanxi province,' says Bando. 'And we were stuck in a terrible traffic jam. Oliver's belly was hot and distended. We had given him antibiotics and pain medication but we were at least two days from being back in Chengdu and if the bear had peritonitis he was not going to make it.'

The convoy was given a police escort to a private hospital, the biggest in the region.

'We did the operation on the back of the truck,' says Bando. 'We needed oxygen cylinders, which the hospital provided.'

Oliver was opened up by Bacon as Bando monitored his breathing. The spectacle attracted a huge crowd. The other staff had to form a cordon to stop people from getting too close. The two surgeons had to work on their knees.

'The doctors stood at the front of the truck watching us,' Bando recalls. 'They were amazed because the equipment we had for our mobile facility was better than the equipment they had for their human patients, which was tragic.'

Oliver's operation was a success and the team arrived back in Chengdu to a hero's welcome.

The China Bear Rescue Sanctuary has become famous throughout Sichuan. After the 2008 earthquake, members of staff went into the epicentre and vets used their skills on human victims. Once they had helped as many people as they could, the vets set about rescuing dogs and cats. Their efforts helped strengthen the bonds between the Western staff at the centre and the locals who work alongside them; 151 of the 162 employees at the sanctuary are from Longqiao and adjacent villages. They take care of 175 bears and their enclosures by cleaning, feeding and observing any changes in their behaviour.

In the hours immediately after the earthquake, the sanctuary's staff were warned that a dam upstream of the river that runs alongside the 10-hectare site was likely to burst. If it had done so, the sanctuary would have been submerged.

'All the local staff wanted to stay,' Robinson says. 'I insisted that the staff leave but they wanted to stay for the bears. In the end, we allowed single men to stay but any females and anyone with a family had to leave.'

IN THE DAYS FOLLOWING their return from Shandong, the vets began to patch up the 10 bears. All had diseased gall bladders and all needed to have them removed. Monkey's turn arrived on June 1.

Three hours into the operation, Bando is still trying to remove adhesions from inside the bear's abdominal cavity so that she can assess whether the animal's liver is cancerous and safely remove the gall bladder.

'These are hernias,' says Bando, pointing to nodules protruding from beneath Monkey's liver. 'She suffered constant insults to her abdominal wall and these hernias could twist and strangulate and kill her. This is a very stressful surgery.'

A nurse holds open a coin-shaped piece of the bear's skin, suspended from which is a section of the abdominal wall and the gall bladder. Bando used scalpels and scissors to remove the adhesions, chunks of tissue that have formed around incisions made by the farmer as it became increasingly difficult for him to jab the catheter into the bear's gall bladder. As difficult as Monkey's surgery is, it does not constitute the worst case from the Shandong farm.

'Bear No269 we named Ali Baba,' Bando says, her face streaked with a red stripe from a geyser of blood that shot from Monkey's abdomen. 'I could not put my hand around the gall bladder, it was so thick and hard and the cystic duct [which carries the bile to the intestine, where it digests fat] was unbelievably abnormal.'

Bando discovered a gallstone stuck in the cystic duct and Ali Baba's bladder was full of infected bile.

'He must have been in unimaginable, excru- ciating pain,' she says. 'The animal's teeth were also smashed in.'

Breaking the bears' teeth is a common practice. The farmers fear being bitten so they either smash the teeth with shovels or cut them to the gum line when the bears are young. The result is that the nerves in the pulp cavities are left exposed and create a further source of agony.

Robinson's campaign against these cruelties has helped reduce the demand for bear-bile products in some parts of the mainland - the closure of the Shandong bear farm made that province the 20th to be bear-farm free - but new sources of custom have sprung up.

In Japan, bear bile is an ingredient in a popu- lar heart tonic pill called Kinshintan (which is freely available in Hong Kong) and in Jilin and Heilongjiang provinces, bear bile is now being marketed aggressively as a hangover cure and a means to protect the liver from the damaging effects of binge drinking, which is commonplace at banquets. In these cases, Robinson has had to adopt a different approach, trying to persuade the government and consumers that bear-bile products are far more likely to do them harm than good.

'The majority of bears that die, die of liver cancer,' says Robinson. 'We believe that there is a connecting factor between the contaminated bile duct and the liver cancer these bears have. If the contaminated bile is killing the bears, what must it be doing to the consumer? People think they're buying bile but what they're getting is pus, bacteria, faeces, urine and rust.'

Monkey's surgery reaches its final stages after seven hours, when Bando pulls out the gall bladder. Bacon lays it down on tissues to photograph and measure it. She begins to dissect the bladder and a thick, black goo oozes out - the normal colour of bile is light green - accompanied by the stench of putrefaction. As she cuts deeper, she finds gallstones and lesions, both of which would have caused Monkey, trapped in her cage, torment on a par with the mythological agony of Prometheus, a Greek Titan whose liver was pecked at daily by an eagle to punish him for stealing fire from the gods.

Monkey's mistake was being in the wrong place at the wrong time - but at least she didn't lose limbs in a trap, as many of the recovering bears in the sanctuary did.

The bears that survive surgery are fed and watered, their medical needs are attended to by professional staff and they can play at will, protected by security cameras and sturdy fences, erected to keep them in but also to guard them from bear farmers and animal rustlers.

After Monkey's procedure, the air outside the operating theatre smells sweet and is full of birdsong. As the sun begins to set, the bears are enticed back into their dens, where they sleep on metal scoops suspended from the walls, their paws splayed over the sides. A few of them still play in the soft light, reluctant to end their hunt for butterflies or forgo one last dip in the pool.

Beyond the enclosures, at the side of the river, there is a small clearing where dozens of bear cages lay piled one upon the other, their narrow spaces seeming to echo with long-ago screams of pain that reach from the dusk into the dark night.

Take a few more steps and there is another clearing, where the bears that died in surgery or soon after are buried. Alongside the rushing river there are dozens of traditional Buddhist burial mounds adorned with crosses. In the earth below lie more than 100 moon and brown bears who can no longer be helped by Robinson and her team. Mercifully, they cannot be tortured and abused any more, either.