BBC correspondent in Dhaka
Legal proceedings have begun in London to determine whether a British organisation was negligent in failing to detect arsenic in Bangladeshi water supplies.
The WHO says up to 80 million Bangladeshis may be affected
Lawyers will argue at a preliminary hearing that the British Geological Survey (BGS) does have a case to answer.
The World Health Organisation has described the naturally occurring arsenic as the largest mass poisoning of a population in history.
It warns that up to 80 million people may be affected.
The proceedings are being brought by two Bangladeshi residents, Binod Sutradhar and Lucky Begum, who allege that BGS' failure to detect high levels of arsenic in ground water endangered their lives.
The pair were diagnosed as having initial symptoms of arsenocosis, a disease that ultimately affects the heart, lungs and kidneys.
It is alleged that BGS was negligent in not testing for arsenic when it conducted a pilot project assessing ground water in central and north-eastern Bangladesh in 1992.
A spokeswoman for BGS said it would vigorously contest the allegation and would argue that it had no case to answer.
For many people in Bangladesh it can sometimes be a choice between death by arsenic poisoning or death by diarrhoea
Timothy Claydon of Water Aid
She said BGS had no responsibility to test for arsenic as the research was part of a small-scale irrigation project that had nothing to do with drinking water.
"The report does not comment on whether the water is safe to drink and does not carry out any assessment on the presence of arsenic because that was not part of the remit," the spokeswoman said.
The case is being closely watched by hundreds of Bangladeshis who say they have also contracted arsenic poisoning and are contemplating similar legal action.
Meanwhile, millions of people in Bangladesh face a terrible dilemma.
"For many people in Bangladesh it can sometimes literally be a choice between death by arsenic poisoning or death by diarrhoea," says Timothy Claydon, country representative of Water Aid.
"People here can't drink surface water because it's bacteriologically contaminated, but they also can't drink tube-well water because in many areas it contains naturally occurring arsenic often over three times higher than the UN recommended level."
Aid agencies disagree over the best course of action.
Some argue the battle for clean water should concentrate on cleaning dirty surface water, which causes diarrhoea and kills an estimated 20,000 people a year compared to only around 13,000 proven cases of arsenic poisoning.
But others argue the priority must be given to the battle against arsenic poisoning.
Because the condition can take 10 years before it is diagnosed, it may eventually kill many more people, they argue.