A water firm has warned Sussex is still in the middle of a drought, despite rising reservoir levels.

Southern Water has received calls from customers who believe there is no longer a shortage because April brought extra rainfall and reservoirs have been filling up.

In fact, the situation is worsening as in some cases reservoirs have been filled using water from rivers, depleting the resources available elsewhere.

Less than a third of tap water in Sussex comes from reservoirs. The rest comes from groundwater in the aquifer, the porous chalk beneath the earth's surface.

Hosepipe bans are still in force across the region and will be for the foreseeable future.

Southern Water has also applied for a drought order to limit water used at sports grounds, to fill swimming pools and wash vehicles and windows.

Meyrick Gough, Southern Water's water planning and strategy manager, said: "What you see is not what you get. Reservoirs account for only 30 per cent of the water we consume. The other 70 per cent is provided by water you can't see, water pumped from aquifers below the ground.

"These in some cases are nearing an all-time low level of water. In other cases they're well below average."

In some areas, water supplies are totally dependent on underground sources.

Mr Gough said the appearance of reservoir levels could be misleading. Southern Water's four reservoirs presently contain about 80 per cent of their combined capacity. Weir Wood Reservoir has risen from 59 per cent only a couple of weeks ago to 65 per cent.

Mr Gough said: "At this time of year they would normally be nearly 100 per cent full. In fact, the April reservoir water levels are lower this year than any other April during the past ten years."

He said there had been partial recovery since January when the level at some reservoirs was as low as 35 per cent but the April rain was not responsible.

It is because Southern Water has been filling the reservoirs by pumping water into them from rivers.

Mr Gough said: "April rainfall so far has been no more or less than we expected. Although welcome, it comes on the back of two consecutive dry winters. That presents us with a growing problem this spring and summer. With spring approaching, vegetation begins to develop and soil starts to dry out. During spring and summer any rainfall we experience has an extremely minimal effect on restoring water supplies. What we call our recharge season is now nearly over.

"The next recharge season won't begin until late October - which means we must manage until then with the water we have now.

"Overall levels are still low. That's why the hosepipe ban is imperative, that's why we have applied for powers which will allow us to introduce further restrictions and that's why it's vital we are all as frugal as possible in use of water the rest of this year."

Ardingly Reservoir, owned by South-East Water, is full for the first time this year, compared to 52 per cent in December.