Level four comes with a warning that illness and death may occur among the fit and healthy.
The Met Office have warned of the exceptionally hot spell 'leading to widespread impacts on people and infrastructure'.
People have been told to expect adverse health effects not limited to those vulnerable to extreme heat, a high risk of heat-sensitive systems and equipment failure, as well as delays and closures on roads.
Met Office spokesman Grahame Madge said: “If people have vulnerable relatives or neighbours, now is the time to make sure they’re putting suitable measures in place to be able to cope with the heat because if the forecast is as we think it will be in the red warning area, then people’s lives are at risk.
“This is a very serious situation.”
As part of the warning, the Met Office have also stated 'substantial changes in working practices and daily routines will be required'.
Mike Childs, head of science, policy and research at environmental organisation network Friends of the Earth, said: “Each year the effects of climate breakdown are becoming more evident and more severe.
“Without meaningful Government intervention millions of Brits, particularly older people and young children, will be at increasing risk from health-threating heatwaves like the one we’re experiencing.”