The chemicals were traced in an investigation by BBC Radio 4's Face The Facts programme and prompted scientists to call for the practice to be halted immediately.

However, Vice-President of Coca-Cola in India, Sunil Gupta, denied the fertiliser posed any risk.

"We have scientific evidence to prove it is absolutely safe and we have never had any complaints," Mr Gupta said.

Face The Facts presenter John Waite visited the plant following complaints from villagers that water supplies were drying up because of the massive quantities of water required by Coca-Cola.

Villagers, politicians, environmentalists and scientists have accused the firm of robbing the community of the area's most precious resource.

They say the area's farming industry has been devastated and jobs, as well as the health of local people, have been put at risk.

'Disturbing'

As part of the probe, Face The Facts sent sludge samples to the UK for examination at the University of Exeter.

Tests revealed the material was useless as a fertiliser and contained a number of toxic metals, including cadmium and lead.

The lab's senior scientist, David Santillo, said: "What is particularly disturbing is that the contamination has spread to the water supply - with levels of lead in a nearby well at levels well above those set by the World Health Organisation."

According to Britain's leading poisons expert, Professor John Henry, consultant at St Mary's Hospital in London, immediate steps should be taken by the authorities in India to ban the practice immediately.

The levels of toxins found in the samples would, he said, cause serious problems - polluting the land, local water supplies and the food chain.

"The results have devastating consequences for those living near the areas where this waste has been dumped and for the thousands who depend on crops produced in these fields," Professor Henry said.

'Good for crops'

Cadmium is a carcinogen and can accumulate in the kidneys, with repeated exposure possibly causing kidney failure.

Lead is particularly dangerous to children and the results of exposure can be fatal. Even at low levels it can cause mental retardation and severe anaemia.

Professor Henry said: "What most worries me about the levels found is how this might be affecting pregnant women in the area. You would expect to see an increase in miscarriages, still births and premature deliveries."

Mr Gupta said local farmers had been grateful for the fertiliser because many could not afford brand-name products of their own.

"It's good for crops," he said. "It's good for the farmers because most of them are poor and they have been using this for the past three years."

Coca-Cola say they will continue to supply the sludge to farmers.