Babar Kabir, head of the bank's Water and Sanitation programme, told Reuters that tests on another 30,000 wells also showed arsenic in nearly 40% of the wells.

The tests, carried out by the United Nations Development Programme (UNDP), ended last week, and represent a first step towards tackling a crisis which has developed in the past decade.

According to some reports, arsenic poisoning could threaten 75 million of Bangladesh's 120 million population, mostly in the north and west of the country.

The wells - of a type known as tubewells - provide water for 90% of the population.

They were introduced by aid agencies, intended as a reliable source of clean ground water in a country where surface water is often contaminated by disease.

But naturally-occuring arsenic in the earth appears to have contaminated the water which collects in the wells.

Deadly poison

Red paint is used to mark an unsafe well

If taken in sufficient quantities, arsenic can cause serious skin conditions, tumours and breathing difficulties, which can lead to death.

The World Bank announced in August that it would provide $30m dollars in aid to Bangladesh to combat arsenic contamination in its water supplies.

Mr Kabir estimated that Bangladesh would eventually require $275 million in the next 10 to 12 years to fight the problem.