The government is urging people to save water as it warns action is needed to prevent severe drought conditions becoming "the new normal".
Water and flooding minister Emma Hardy told MPs current drought conditions "is something we have not experienced for decades" and urged people to "think about ways to use water wisely over the coming weeks".
Severe drought was declared across parts of southern England on Thursday after months of low rainfall.
Parts of Cornwall, Devon, Dorset, Somerset, East Sussex and West Sussex are all affected by the move announced by the Environment Agency.
The government said supplies would not be interrupted but declaring severe drought would ensure "proactive steps are taken to safeguard water before any disruption occurs".
Speaking in the House of Commons, Hardy said the measures were down to "exceptional low rainfall" and depleted reservoir levels.
"This has been the lowest rainfall in over a century," she said.
Robbie Moore, shadow minister for environment, food and rural affairs, accused the government of being "slow to react" to the conditions.
"This is the third drought in five years, it should not have caught the government off guard," he told MPs.