The South East was hit by a two-year drought and hosepipe bans

South East Water set up a plant in Newhaven to test the process of turning seawater into drinking water, but found it was "an expensive option".

Engineering studies to reach additional supplies underground in aquifers in Kent and Sussex are now under way.

The firm said the sources could deliver an extra 9.5m litres of water per day.

The natural aquifers are at South Heighton and Kemsing, which is north of Sevenoaks.

Water-efficiency

The process tested at Newhaven involved filtering water and then forcing it through a membrane at high pressure to remove salt and other pollutants, creating pure, clean water.

The expense of desalination was down to operating costs and also the environmental costs of disposing of concentrated salt water, David Shore, operations director, said.

It was found that desalinated water would cost �450 per million litres, compared with �50 per million litres for abstracting and treating river water, and �35 per million litres for abstracting and treating ground waste

Mr Shore said the other option was to develop the additional resources as well as fitting meters and encouraging people to become more water-efficient.

"We are required by both our economic and environmental regulators to pursue the least-cost, most sustainable option when finding new ways to provide water, as ultimately our customers pay," he said.