The "winter drought" across the UK could have "serious consequences", the Environment Agency says.
Water firms are hoping for more rain for winter and spring
The drought was at its "most severe" in Kent and Sussex, where some areas had seen their driest conditions since the 1933-34 drought, the agency said.
The rest of south-east England is the driest since 1976, when hosepipe bans were in place, a spokeswoman added.
The agency warns of water restrictions and environmental impacts if rainfall stays below average.
The spokeswoman said: "If the rest of winter and spring remain below average in rainfall, we will expect to see restrictions on water use over much of the South East in summer."
Hosepipe and sprinkler bans, more drought permits and drought orders to help water companies manage supplies may be needed, she added.
She also warned the dry weather could have an impact on the environment in the areas affected.
"As the drought progresses, we can expect to see deteriorating water quality in some rivers and fish kills," she said.
Meanwhile, Southern Water said sprinkler and hosepipe bans in the parts of Sussex and Kent it covers were set to remain in place this summer.
Jessica Spratt, a spokeswoman for the firm, said the problem was that water supplies had not been properly recharged during a dry winter.
Two of its reservoirs, Weir Wood and Bewl Water, are just over a third full.
Last week, the Environment Agency granted the firm a drought permit allowing it to take more water from the river Medway, after Bewl Water reservoir in Kent reached its lowest level since it was built in the 1970s.
Between November 2004 and June last year, Sussex and Kent saw eight consecutive months of below average rainfall.
Miss Spratt said Southern Water was a "long way off" measures such as introducing standpipes - a measure seen in some parts of the UK during the 1976 drought.
"Our next measure would be stopping non-essential use of water, which would affect companies," she said. "Things like car washes, and watering cricket grounds or sports fields.
"But we will have to wait to see what happens and we are doing everything we can to prevent this."
She added that the public had responded well to publicity campaigns introduced last summer on how to save water.
Meanwhile, the Met Office, said February, which it counts as the last winter month, was statistically the driest of those months.
"If you look at the long term averages from 1970 to 2000, the wettest months are December and January, and it tends to be a bit lower in February," said a spokesman.