They are supposed to be sacred animals. Revered above all other beasts by Hindus - ranked as high as Brahmin priests, the "twice-born", for their sweetness and generosity - cows still tramp the streets of most Indian towns and cities, mingling with the traffic, nosing through the rubbish skips in the markets, roaming deserted highways at night.

They are huge but very docile. The native breed is creamy white in colour, with a distinctive hump. Sometimes a pious Hindu can be seen feeding a roadside cow with a carrot or chappati. Rarely are they the butt of anger or impatience.

And a fleeting appraisal from the comfort of a tour bus might suggest that India's cows have survived the country's patchy modernisation unscathed. But a campaign to be launched tomorrow by People for Ethical Treatment of Animals (Peta), backed up by The Independent's own investigation, reveals the Indian treatment of its holiest animal as a scandal of cruelty, greed and corruption.

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The cow's special status in India is enshrined in law. With the exception of two states, the slaughter of cows and calves is totally forbidden, whatever the reason and at whatever age. Bulls and bullocks and she-buffaloes are protected up to 15 years of age.

The arrival of Hindu nationalists in power both at the centre, where they are the leading party in a coalition, and in a number of states, has also enhanced the protection which cows receive. Between 1995 and 1999, the Hindu nationalist Bharatiya Janata Party (BJP) government of Delhi gave 390 acres of land and more than £160,000 for the setting up of gosadans or shelters for cows.

But all this apparent reverence and protection masks a trade in cows and cow products which involves unbelievable barbarity and cruelty.

Much of the abuse stems from the fact that the trade in and slaughter of cows is almost entirely clandestine and illegal - but the authorities which should be stopping it are routinely bribed to let it continue. There is, therefore, no scrutiny or regulation of the trade anywhere along the line.

Although Hindus hold the cow in special esteem, and Jains regard all life as so sacred that they try to avoid hurting insects, investigations show that all India's major communities are complicit in the cruel treatment of cows.

Hindu farmers allow their cows to be taken for slaughter. Muslims butcher them using primitive techniques in appalling conditions. Hindus, Jains, Sikhs, Muslims and Christians all profit.

And because much Indian beef finds its way to the Middle East and Europe from Kerala and Bangladesh - "we took up a lot of the slack from Britain caused by mad cow disease," says one authority - and leather products made from Indian cow hide are sold in High Street shops such as Gap, the British consumer is also unknowingly benefiting from the abuse.

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Thanks to the lobbying of Hindu nationalists, the slaughter of cows has been banned in all Indian states and territories except West Bengal, in the north-east, and Kerala in the far south. One result of this is secret, hole-in-the-wall cow abattoirs dotted around the country, especially in Muslim quarters of towns and cities. But the main result is an appalling traffic of cattle.

"There is a huge amount of trafficking of cattle to both West Bengal and Kerala," said Mrs Gandhi, Minister of Social Justice and Empowerment in the present government and a veteran campaigner against animal abuse of all sorts. "The ones going to West Bengal go by truck and train and they go by the millions. The law says you cannot transport more than 4 per truck but they are putting in up to 70. When they go by train, each wagon is supposed to hold 80 to 100, but they cram in up to 900. I've seen 900 cows coming out of the wagon of a train, and 400 to 500 of them came out dead."

The trade exists because of gross corruption, Mrs Gandhi says. "An illegal organisation called the Howrah Cattle Association fakes permits saying the cattle are meant for agricultural purposes, for ploughing fields or for milk. The stationmaster at the point of embarkation gets 8,000 rupees per train-load for certifying that the cows are healthy and are going for milk.

"The government vets get X amount for certifying them as healthy. The cattle are unloaded just before Calcutta, at Howrah, then beaten and taken across to Bangladesh by road. Bangladesh, which has no cows of its own, is the biggest beef exporter in the region. Between 10,000 and 15,000 cows go across that border every day. You can make out the route taken by the trucks by the trail of blood they leave behind."

Even more horrifying is the transport of cows to the abattoirs on the border of Kerala in the extreme south of the peninsula. Mrs Gandhi says, "On the route to Kerala they don't bother with trucks or trains: they tie them and beat them and take them on foot, 20,000 to 30,000 per day." All Kerala's slaughter houses are on the border. "Because they have walked and walked and walked the cattle have lost a lot of weight, so to increase the weight and the amount of money they will receive, the traffickers make them drink water laced with copper sulphate, which destroys their kidneys and makes it impossible for them to pass the water - so when they are weighed they have 15kg of water inside them and are in extreme agony."

Ingrid Newkirk, President of Peta, followed one of the caravans of cattle stumbling towards Kerala. "It's a hideous journey," she writes in the forthcoming issue of Animal Times, Peta's journal. "To keep them moving, drivers beat the animal across their hip bones, where there is no fat to cushion the blows. The cows are not allowed to rest or drink. Many cows sink to their knees. Drivers beat them and twist their battered tails to force them to rise. If that doesn't work they torment the cows into moving by rubbing hot chilli peppers and tobacco into their eyes."

When they finally make it to the slaughterhouses that stand on the Kerala border, the end they confront is unspeakable, Mrs Gandhi says. "In Kerala they also have a unique way of killing them - they beat their heads to a pulp with a dozen hammer blows. A well-intentioned visitor from the West, trying to improve slaughterhouse practice in Kerala, exhorted them to use stun guns, saying that the meat of an animal killed in this fashion (rather than having its throat slit) tasted sweeter. The stun guns that she left behind quickly broke and fell into disuse, but the belief that the meat was sweeter took hold - which explains this horrible method of slaughtering."

The sentimental attitude towards animals prevalent these days in the West is alien to traditional India, as to the rest of Asia. But respect and reverence for all life is fundamental to Hinduism - most Hindus are vegetarians even today - and the prevailing attitude is enshrined in the Gandhian word ahimsa, "do no harm".

Yet greed, poverty, ignorance and absence of regulation and supervision have brought India's cows to the point where their treatment is on the threshold of becoming a major international scandal.

At root it is a political issue. The ban on cow slaughter has been a fundamental plank of the Hindu nationalists for many decades - but a plank with which to bash cow-eating and cow-slaughtering Muslims, not to improve the lot of the actual cows. The apparent beneficiaries of the agitation, the cows, were of mainly symbolic importance.