The British Geological Survey is facing a lawsuit for overlooking arsenic in tests on Bangladeshi groundwater. The suit could cost the BGS £30 million - almost its entire annual budget.
Papers to begin proceedings on behalf of 2,000 Bangladeshis are likely to be served by December.
In 1992, the BGS screened groundwater in Bangladesh, where wells have provided drinking water since the 1970s because surface water was contaminated. The geologists did not test for arsenic because, they said, there was no reason to suspect it.
But within three years, it emerged that millions of people had been exposed, and are still being exposed, to arsenic-tainted water. One scientist said the situation was worse than Chernobyl.
Bozena Michalowska, a solicitor working on the case for London-based legal firm Leigh Day, said some scientists had raised suspicions in the late 1980s. "If the BGS had tested for arsenic in its survey in Bangladesh, it would have found it. Not doing so was inexcusable," she said.
Ms Michalowska and senior solicitor Martin Day will represent up to 2,000 people living in areas tested by the BGS who have developed arsenic-related conditions such as gangrene and tumours. She said the firm will seek damages for personal injury averaging £15,000 a person. If the suit succeeds, the BGS could have to pay up to £30 million.
The BGS dismisses the claim. It said in a statement: "BGS does not accept that they can be blamed for the suffering which has occurred, and any legal claims, which we regard as wholly misconceived, will be resisted."
The BGS tests searched for 36 elements and compounds, but arsenic was not among them as it was not at the time associated with the geology of Bangladesh.
John McArthur, professor of geochemistry at University College London, has since found that bacteria living off peat buried in the flood plain may release arsenic from tiny particles of sediment.