The heat health warning has been increased to a “national emergency” level four amid predictions the mercury could hit a record 40C.
The current heatwave is set to peak on Tuesday, with an 80% chance of the temperature topping the previous high of 38.7C (101.7F) set in Cambridge in 2019.
The Met Office has warned people’s lives could be at risk and has issued its first red warning for extreme heat on Monday and Tuesday.
The concern is so serious that ministers will hold an emergency Cobra meeting on Saturday to discuss the escalating heatwave.
Transport for London (TfL) has also advised passengers to only travel for “essential journeys” on Monday and Tuesday.
What are the five heat levels and what happens when they are triggered?
The UK Health Security Agency’s (UKHSA) heat health alert system is made up of five levels.
Level zero: Long-term planning to reduce risk from heatwaves
The planning takes place year-round to reduce the harm caused by heatwaves when they arise.
This includes ensuring that urban spaces like housing, transport systems and workplaces are kept cool.
Level one: Heatwave and summer preparedness
The minimum state of vigilance used during the summer months encourages social and healthcare services to plan closely ahead.
Services make sure preparedness work is ongoing, and the heatwave plan remains at this level between 1 June and 15 September unless a higher alert is triggered.
Watch: Is the UK heatwave a cause for concern?
Level two: Alert and readiness
This comes into force when the Met Office forecasts a 60% chance of temperatures being high enough on at least two consecutive days to have significant effects on health.
“This is an important stage to ensure readiness and swift action,” the UKHSA says.
Level three: Heatwave action
We move into level three once the Met Office confirms threshold temperatures have been reached in any one region or more, and when specific actions targeted at high-risk groups are required.
Level four: National emergency
Level four is reached “when a heatwave is so severe and/or prolonged that its effects extend outside the health and social care system.
At this level, illness and death may occur among the fit and healthy, and not just in high-risk groups”, the UKHSA says.