Water companies have also urged people to use water responsibly.
South East Water said on Thursday morning that parts of Whitstable and Headcorn in Kent, Peacehaven in Sussex and Compton in Surrey have no water or low pressure.
The Met Office urged the public to help protect vulnerable people, including the elderly and those with underlying conditions.
“The heat will be accompanied by high humidity, exacerbating the potential for discomfort and health impacts,” it said.
Scientists are warning even hotter temperatures are likely in the years ahead.
They have described the records as “extraordinary” and say the UK needs to make preparations for more extreme heat a much greater priority.
