Slum-dwellers in Dar es Salaam pay the equivalent of £4 ($8, €5) for 1,000 litres of water, bought over time and by the canister. In the same Tanzanian city, wealthier households connected to the municipal supply receive that amount for just 17p. In the UK, the same volume of tap water costs 81p and in the US it is as low as 34p.

Figures from other countries confirm the evidence: it is generally the poorest who pay most for what is one of the most essential of all natural resources. Water is in short supply for a large proportion of the world's people: about 1bn lack access to clean water and 2.6bn have no sanitation. An estimated 5,000 children die every day from water-related disease, according to WaterAid, the London-based charity.

If the number of people lacking safe drinking water were halved, at a cost of about $10bn, the world would benefit by $38bn in annual economic growth, according to the United Nations Development Programme. Disputes over water rights can, the UNDP argues, lead to conflicts - such as in Darfur.