The environment across the south is beginning to show real signs of the onset of drought, the Environment Agency Board was told on Wednesday, with low groundwater levels and low river flows setting in.

And in this situation, some water companies may be leaving it too late to impose restrictions on consumers, the Board heard. People should expect more hose pipe bans across southern England this summer, Environment Agency chief executive Barbara Young warned.

Barbara Young said: "Some water companies are reluctant to impose demand restrictions. The Environment Agency is monitoring companies' actions very closely. We believe they must take adequate and prudent steps sooner rather than later in order to reduce more far reaching risks to public water supply if the drought continues, and to avoid any unnecessary impact on the environment."

Southern England has experienced one of the driest winter and spring periods in the last hundred years – in some places only the winter of 1975/76 was drier. Hampshire, Sussex, Kent and the Thames Valley have had only about two-thirds of average rainfall since November 2004.

The Agency is waiting to hear the outcome of the country's first Drought Order application this year, under which Southern Water has asked permission to halve the normal compensatory water flow from the reservoir into the River Medway. The Agency has objected to the full cut, insisting that a minimum flow must be maintained to protect the sensitive habitat and wildlife species of the river from damage. The Secretary of State for Environment is due to decide the application soon.

Drought setting in

As the dry weather continues, the environmental impacts will become more evident, the Board heard. In addition, the impact of summer storm sewer overflows combined with increased temperatures on water quality and fish mortality in the Thames, is already becoming evident this year, underpinning the case for the Thames 'super sewer', Barbara Young told the Board.