South East Water has asked homeowners to take short showers instead of baths over the Christmas period after issuing a drought order.
Residents in 65,000 homes in Sussex have also been asked not to wash their cars with hosepipes, to turn off the taps while they are cleaning their teeth and install water displacements bag in toilet cisterns.
The drought order, which was granted by Caroline Spelman on 21 December, is the first time such emergency measures have been introduced in winter since December 2003.
The order gives South East Water, which also serves Kent, Surrey, Hampshire and Berkshire, permission to reduce the minimum flow in the River Ouse, below the Barcombe Water Treatment Works.
A statement on the firm’s website said the move: “…allows the company to immediately reduce the minimum flow in the River Ouse, below its Barcombe Water Treatment Works, from 20 million litres a day, to 5 million litres per day. The Drought Order is valid up to the 31st March 2012.
“This will help ‘slow down’ the rate of water being drawn off at Ardingly Reservoir, enabling it to re-fill more quickly this Winter, to help secure customers’ supplies ahead of next Spring and Summer.”
Lee Dance, Head of Water Resources and Environmental for South East Water, said: “We rely on winter rainfall to recharge our underground water sources and reservoirs to take us through to the warmer spring and summer months.
“The Sussex area has seen an increase in rainfall during December, which has certainly helped the current water resource situation, but it by no means solves the problem. We would still hope to see prolonged periods of rainfall over the coming months in order to bring our water resource levels back to ‘normal’ for this time of year.”
Drought orders allow water firms to impose hosepipe bans and other restrictions, but the company insists this is not yet necessary yet.
Meanwhile the Environment Agency has been asked to investigate South East Water over mis-measurements of the reservoir which contributed to the need for the drought order.
But the government said South East Water's mis-measurements of the Ardingly Reservoir, which is now 30% full thanks to rain in December, delayed its application for a drought order and led to it being needed more urgently.
The company admitted today that it had overestimated the amount of usable water in the reservoir.
A Defra spokeswoman said: "The government has acted quickly to protect the water supply to homes served by Ardingly reservoir by granting a drought order to South East Water."
"An exceptional lack of rain over the last eight months caused the deficiency of supply.
"However South East Water has acknowledged that their mis-measurement of the water in the reservoir delayed their application for a drought order and contributed to the urgency with which it is now needed.
"The Environment Secretary has asked the Environment Agency to commission an independent review into how this situation came about and we expect the company to co-operate fully with it."
In a statement, South East Water said that following a dry spring this year the company implemented its drought plan.
"Normally we would expect rainfall in the early Autumn to replenish our reservoirs after the Summer, but an exceptionally dry September, October and November raised the urgency of the situation.
"South East Water monitors its reservoir levels, but during its more detailed investigations as part of its drought planning it found the relationship between levels and volume led to an overestimation of the storage available.
"This had not been an issue until the extended, dry autumn when, due to the conical shape of the reservoirs, the variance between the actual depth of water - and what volume of that water is usable for public water supplies - became more apparent as levels significantly dropped at Ardingly."
The company said it was now reporting the figures for the usable volume of water to the Environment Agency and Defra.
An Environment Agency spokeswoman said: "We support the Secretary of State's decision to grant a drought order which will help to safeguard the water supplies for South East Water's customers and the local environment come next spring and summer.
"We will now be conducting a full review into how and why a drought order has been required."