The threshold varies from 25C across the north and west of the UK, to 28C in parts of England.
In Suffolk, where the threshold is 27C, a temperature of 29.3C was recorded in the village of Santon Downham on Thursday. This was the third day in a row the county’s threshold had been breached – resulting in a heatwave being declared.
In London, where the threshold is 28C, a heatwave could be declared on Friday, as the temperature has now passed that figure for two days in a row.
At Royal Ascot, which takes place in Berkshire, outside of London, race organisers confirmed dozens of attendees required medical assistance because of heat-related illness.
The alert system works in conjunction with the Met Office but has a focus on health risks using four levels of warning: green, yellow, amber and red.
An amber warning means the whole health service is likely to be affected by the hot weather.
Among examples given by UKHSA are difficulties managing medicines, the ability of the workforce to deliver services and internal temperatures in care settings exceeding the recommended thresholds.
The agency also refers to a possible rise in deaths – particularly among those aged 65 or over or with health conditions – as well as health risks to the wider population.
