Three reservoirs in England now have "exceptionally low" water levels – with a further seven "notably low" as the country is on track for the driest July on record.
Reservoir storage currently stands at 72%, down from 75% a week ago and nearly 10 percentage points below the long-term average for this time of year.
The reservoirs now classed as having "exceptionally low" levels are Blagdon and Wimbleball, both in Somerset, which are 56% and 54% full respectively, and Hanningfield in Essex, which is 65% full.
The seven with "notably low" levels include Ardingly in West Sussex, which is 66% full; Blithfield in Staffordshire (63%); Clatworthy in Somerset (56%); and Clywedog in Powys (69%), which provides water for central and north-west England.
Also in this category are the Dove group of reservoirs in Derbyshire (77%); Grafham in Cambridgeshire (78%); and the Lower Thames group of reservoirs in Berkshire and west London (74%).
The figures are from the Environment Agency's latest weekly dry weather and drought report.
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The Met Office said on Friday that a prolonged heatwave, exceptionally low rainfall, and widespread drought conditions have combined to make July "a remarkable month".
England and Wales are both on course for their driest July since rainfall records began in 1836.
Southern England has been particularly dry, with just 1.5mm of rainfall - which is 2% of its long-term average rainfall for the month.
Northern Ireland is on track for its eighth driest July while the UK as a whole is on course for its third driest.
Scotland has received 72% of its average rainfall for the month and is not currently within its top 10 driest Julys.
Forecasters have said temperatures in the coming days will be closer to average for this time of year.
Sky News weather producer Kirsty McCabe said the UK and Ireland can expect "cooler and fresher conditions with a few showers around, mainly in the North".
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