Even in the dog days of summer, the quiet paddy fields that mark the border between India and Bangladesh look as supple and green as the soft stems of herbs grown in a window box. But the daytime tranquillity belies a stark reality. This delta region of the Ganges river is a place of often deadly conflict that underpins an activity many in India would rather not discuss. Every year, hundreds of thousands of cows – considered sacred in India, with export of the beasts banned – are illegally smuggled into Bangladesh where they are turned into shoes, belts, bone china crockery and, of course, meat.

"There is smuggling here every day," said Umesh, a member of a three-man Indian Border Security Force (BSF) team on duty at a watchtower near the village of Kaharpara, just a few hundred yards from the Bangladesh border. "The smugglers will take 50, 100 or 200 cattle at a time. We try to create an ambush and surround the smugglers."

The story of the annual smuggling of an estimated 1.5 million cattle says much about modern India – about the sometimes hypocritical treatment of supposedly sacred cows, the political power of right-wing Hinduism and the corruption that allows the £320m illegal trade to flourish. But ultimately this story is about supply and demand. Hindu-majority India has an estimated 280 million cows but killing and eating them is legal in only a handful of states. Meanwhile, Muslim-majority Bangladesh, where beef is eaten with relish, suffers from a shortage of cattle. Half of the beef consumed in Bangladesh comes from its large, western neighbour.

The snaking border that divides the two countries runs for 1,300 miles. Here in the Murshidabad district of West Bengal, 150 miles north-east of the state capital Calcutta, large sections of it are unfenced. It is a lure both for human traffickers and gangs from both sides of the border smuggling cows.

Villagers, who claimed not to know any smugglers but appeared to know the intricacies of the operation, said cattle were brought by truck from states across eastern India such as Bihar, Orissa and Jharkhand. Some may even be brought from further away. Despite the effort involved, the mathematics is persuasive. An animal that might sell for £60-£80 in the country's cow-belt hinterland will here fetch £130. Once inside Bangladesh, they could change hands for £225 or more.

"Those buying the cows always look to see how fat it is. They feed them husks from the paddy," said Mohammed Ashraf, a blacksmith who was hammering into shape a glowing curved sickle that locals use to cut the rice crop that is harvested three times a year.

Yet the trade comes with a deadly price. The BSF has been accused of killing hundreds of cattle smugglers, as well as civilians not involved in the trade. A 2010 report published by Human Rights Watch (HRW) suggested that more than 900 people had been killed with impunity by the BSF over the past 10 years. It also said locals claimed some BSF members were complicit with the smuggling and took bribes. This year, an incident in which an alleged smuggler was badly beaten by the security force personnel was captured on video.

"Over the last decade, they used excessive and indiscriminate force, shooting at villagers on suspicion that they were smugglers," said Meenakshi Ganguly, HRW's south Asia director. "While many may have been engaged in cattle rustling, the BSF ignored the most basic principles of protecting the right to life. Instead of arresting suspects, they shot and killed them. The BSF claimed they had to use lethal force as self-defence, an argument hard to believe since the police reports on the weapons recovered usually [refer to] sickles and sticks."

Asked about the allegations, a BSF spokesman said: "The BSF is a disciplined and professional force [and] exercises utmost restraint in the use of any force. The BSF has also an impeccable record of upholding human rights."

Ms Ganguly said that since issuing its report, the BSF had started using rubber bullets which led to a drop in fatalities. But, villagers said their evenings were still sometimes disrupted. "We hear the gunshots at night-time. Sometimes the smugglers get shot. It's mainly people from the other side of the border," said Mr Ashraf. Locals said the smugglers often used teenagers to transport the cattle across the border in the belief the security forces were less likely to shoot a youngster.

There is a clear antagonism between the guards and the villagers. Some locals said the BSF troops retaliated against anyone they could find. Matir Rahaman, a rice farmer who was cycling back from the fields, said he had been badly beaten by BSF personnel. "One night the cows came over the border and the paddy got smashed. I went to the BSF and said, 'Why is this happening'. They said, 'You are smugglers' and they attacked us with [metal-tipped bamboo sticks]," he alleged.

Ashfaqur Rahman, a retired Bangladesh diplomat who now chairs the Dhaka-based Centre for Foreign Affairs Studies, said the matter was sensitive but that legalising the export of cows or beef would put an end to corruption and violence. "There needs to be wise counsel on both sides," he said.

An irony is that India is expected to become the world's largest exporter of beef – from non-sacred buffaloes, rather than cows – by next year. According to an estimate recently published by the US Department of Agriculture, India is likely to export 1.5 million tons of beef in 2012, a 25 per cent increase from last year. Its biggest markets are south-east Asia, the Gulf and Africa.

Cows have been considered sacred in India for centuries, and in only a few states is killing and eating them legal. More recently, a movement by Dalits, or so-called untouchables, demanding the right to eat cows has gathered pace. In 2004, Indian historian DN Jha published the controversial The Myth of the Holy Cow, which argued that during the period when a number of the most important Hindu religious texts were produced, people in India ate cows.

Kancha Ilaiah, a Dalit activist and a professor at Maulana Azad National Urdu University in Hyderabad, believes Aryan invaders of Hindu promoted the (white) cow over the (black) buffalo. "The buffalo predates the Aryans," he said.

There have been attempts by the Indian authorities to review the ban on cow exports. Earlier this year, a report by the government's central planning committee suggested changing the law to allow the export of beef. The plan was hastily dropped and explained away as a "clerical error" amid an angry backlash from right-wing Hindu organisations such as the Vishwa Hindu Parishad and so-called "cow protection" groups.

Among those who complained was the UN-affiliated International Organisation for Animal Protection. The group's India director, Naresh Kadyan, said: "It is the fundamental duty of Indians that [everyone] should respect all animals. We strongly opposed the lifting of the ban and the government made a U-turn," he said. "The cow is a very important animal for Hindus."

Revered and worshipped: Saintly beasts

Elephants

In Thailand, the elephant is considered the national animal, and it is also revered in Burma, Cambodia and Laos.

Particularly auspicious is the white elephant – not a distinct species but an albino or elephant with particularly pale skin – which Buddha's mother is said to have dreamt about before the birth of her son. The appearance of a white elephant in the reign of a monarch or leader is meant to signify good fortune and power.

Cats

The ancient Egyptians took their worship of animals to artistic heights with statues to honour their feline gods, which frequently featured cats' heads on human bodies.

Cats were prized for their useful rat-catching abilities, and some argue they were first domesticated in the region.

While cats are no longer worshipped as gods in modern Egypt, they are certainly preferred as pets to dogs, which are traditionally considered unclean in Islam.

Monkeys

Their association with the Hindu faith – the monkey god, Hanuman, helped Lord Rama defeat the evil king Ravana – has largely protected India's monkeys in the face of much annoyance at their mischievous and sometimes aggressive ways.