The region joined parts of eastern England which have been drought-afflicted since last summer, as two dry winters left some rivers and groundwater supplies at levels lower than 1976.
The drought was declared by the Government as the Environment Department (Defra) convened a summit of water companies, farmers and wildlife groups to discuss potential water shortages in England.
Following the summit, companies in the South East warned that water restrictions such as hosepipe bans may be needed to ensure essential public supplies are maintained throughout the summer.
The spread of drought prompted warnings from green groups that England's water supply system was failing to work effectively to conserve water and protect the environment.
And there were calls for householders to save water in the face of the dry conditions.
The water companies said they were "not running out of water", but without a dramatic improvement in the situation, with significant rainfall in the next few weeks, they will have to implement measures to tackle drought.
The South East has experienced the driest October to January in the region since 1992, with around 73% of expected rainfall, the companies said.
Since October 2010, the region has had only four-fifths of the long-term average rainfall, leading to a "huge cumulative shortfall" in the rain needed to replenish the groundwater aquifers that supply much of the region's water.
The lack of rain continued last month, with the South East receiving two-thirds (66%) of the long-term average rainfall for January.
In the Thames Valley and London, rainfall has been below average for 18 of the last 23 months.
Some companies have already been granted or applied for drought permits or drought orders to refill reservoirs. The worst affected, Ardingly reservoir in West Sussex and Bewl in Kent, are around two-fifths of their normal levels.
In eastern England, Lincolnshire, Cambridgeshire, parts of Bedfordshire, Northamptonshire and west Norfolk are still in a state of drought, which was first declared in those areas last summer.
The Anglia region saw its driest ever September to January period.
A number of rivers in eastern and southern England are at very low levels, with some drying up completely in places.
Speaking after the summit Environment Secretary Caroline Spelman said: "Drought is already an issue this year with the South East, Anglia and other parts of the UK now officially in drought, and more areas are likely to be affected as we continue to experience a prolonged period of very low rainfall.
"It is not just the responsibility of Government, water companies and businesses to act against drought. We are asking for the help of everyone by urging them to use less water and to start now."
Thames Water's sustainability director Richard Aylard said the lack of rain had led to very low levels of groundwater, needed to keep rivers flowing, and things were "going to get worse" without significant rainfall in the next few months.
A recent survey of Thames Water customers revealed almost half (45%) thought it was unlikely or very unlikely the region would face a drought this year, prompting the company to urge people to think about their water use.
Mr Aylard warned: "There is a high chance we will need restrictions at some stage this summer unless either we get a lot of rain or fantastic co-operation from customers using less water."
Thames Water says people can save supplies with simple measures, such as turning off the tap while cleaning their teeth or taking shorter showers, fixing leaks and only washing full loads of laundry.
The RSPB warned that wildlife could be hit by the continuing dry conditions, while the Country Land and Business Association (CLA) called for farmers to get a "fair share" of water to ensure food crops are not affected.
CLA president Harry Cotterell also called for the Government to extend permitted development rights to allow the building of on-farm reservoirs, which would help during droughts in the future.
The Royal Horticultural Society issued advice to gardeners to save water and protect plants in the drought.
The RHS recommended digging in lots of compost or mulch to help retain moisture in soil, putting in new plants when they are small to make them more resilient to dry conditions, mulching in plants after they have been planted, and picking the right plants for particular soil conditions.
PA
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