The Met Office on behalf of the Department of Health has raised its heatwave alert level in parts of England.
London and the South East are now on Level three of four, with temperatures set to reach at least 32C.
The Department of Health has issued special advice for the elderly and other people at risk from the heat.
It is the first time the Met Office has issued a level three alert since June 2006.
It occurs when temperatures reach a certain threshold and are sustained in one or more regions over two nights and the intervening day.
The Met Office said a key factor in raising the alert had been night-time temperatures, with some areas not falling below 18C.
The highest heatwave level, level four, is classed as ‘red emergency’.
It is reached when heat is so severe that illness and death may occur among the fit and healthy, and not just in high-risk groups.
The DH said those at most risk from heatstroke included the very young and the very old, and people with heart and respiratory problems.
Yvonne Doyle regional director of Public Health in the South East said: ‘Everyone’s health can be affected by the heat, with greater risks of dehydration, sunburn, heat exhaustion and heatstroke. Those most at risk are the very young and the very old, and people who already have health conditions, especially heart and respiratory problems – the heat can make their symptoms worse.’
A DH spokesperson said that people should contact their local environmental health officer if they had concerns about living conditions for themselves or a vulnerable friend, neighbour or relative.
People with respiratory problems should stay inside during the hottest part of the day.
Anyone with symptoms of heat exhaustion should drink plenty of water and move somewhere cool. Those with symptoms of heatstroke should be moved somewhere cooler and sponged with water while urgent medical treatment is sought.
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