Health warnings have been issued as Britain continues to swelter in the country's first heatwave for seven years.
After five consecutive days basking in 30C-plus temperatures, forecasters said Britain was in the midst of its first prolonged heatwave since 2006.
The Met Office responded by issuing a "level three heat health watch" for London and the South East and advised "alertness and readiness" for those in the North West and North East.
Level three warnings are only one notch below the most serious alerts and are put out when high temperatures place the very young, the very old and those with chronic diseases at risk.
Forecasters said Wednesday could break the record for the warmest day of the year, with temperatures expected to creep as high as 32C (89.7F) in parts of England.
Sky News weather presenter Isobel Lang said: "Wednesday could be the hottest of the current heatwave across the London area with a high of 32C, topping Heathrow's 31.4C on Monday and Kew Gardens' 31.2C yesterday."
Referring to the level three heat-health warning for London and the South East, she added: "The temperature threshold for the region is for daytime highs of 31 or 32C (90F) and night minima of 16 to 18C (64F) over two days and the intervening night."
The alert comes after a series of tragic accidents in which people have died trying to cool off.
Emergency services have reiterated a warning of the dangers of swimming in open water after the bodies of two men were recovered from the water at a beauty spot in Norfolk where swimming is banned.
There have been four water-related deaths in Wales over the last week. In the latest accident a man in his twenties was reported dead on Wednesday morning after being pulled from the River Ceiriog at Pont-y-Blew, near Wrexham, the previous evening.
In the Brecon Beacons, a 24-year-old man died while swimming at the Cantref Reservoir and a 52-year-old man did not return from his swim in Pontsticill Reservoir in Merthyr Tydfil.
Only days earlier the body of 14-year-old Hollie McClymont from Glasgow was found in the sea near Fontygary in the Vale of Glamorgan. She was last seen in difficulty in the waters near Barry Island.
Two TA soldiers died of suspected heat exhaustion during an SAS assessment in the Beacons on Saturday.
