England's first red alert from the Met Office for Monday and Tuesday also covers southern Scotland.
The warmest day of the year in the UK occurred on Sunday, when temperatures throughout England and Wales rose beyond 30C (86F). A high of 33C was recorded in Flintshire.
On Monday, the temperature might reach 41C, which would be a record high for the UK.
In Cambridge in 2019, the current temperature record of 38.7C was established.
High temperatures triggering national emergency
Elizeu Dias/Unsplash
According to the Met Office, a strange setup caused the jet stream to break up near the coast of Portugal, allowing a whirlpool to flood air from North Africa towards Britain.
The seemingly harmless-looking yet dangerous weather system will move toward Britain as high pressure accumulates and concentrates scorching heat.
Today's temperature will rise gradually before potentially reaching 41C at the beginning of the week, setting off a national emergency.
"On Sunday, temperatures will start to rise throughout most of England and Wales, and this is the first day when temperatures really start to ramp up," said Aidan McGivern, a meteorologist at the Met Office, as per Express.
This has been moving down the Portuguese coast and has contributed to intensifying and concentrating the heat in Spain and Portugal.
As a result, people have seen some extraordinary temperatures.
This vortex will begin to approach through Sunday and the beginning of the week, and on Monday and Tuesday, a plume of extremely hot air will blanket the UK.
The vortex will work in tandem with a big area of high pressure to suffocate Britain and push air toward the ground, which will cause it to heat up quickly.
As it engulfs Britain, it will trap boiling air behind a dome of heat, sending temperatures soaring to 33C and continuing to rise over the course of the following 72 hours.
The hottest temperature recorded in England was 32°C at Nantwich, Cheshire, while Auchincruive, in Scotland's Ayrshire, recorded 26.4°C, as per the BBC.
With a temperature of 27.7C recorded in Armagh, Northern Ireland also saw its warmest day of the year thus far.
Due to the weather on Sunday, there were long lines of cars at Camber Sands in East Sussex and crowded beaches all throughout the nation.
The heatwave is being handled like a national emergency, according to Downing Street.
However, when individuals attempt to cool off, they are advised to use caution when near water.
On Saturday, a teenager drowned in Salford Quays after swimming there, while a guy remains missing after going into a reservoir in West Yorkshire.
Meanwhile, water suppliers cautioned that due to increasing demand, some homes were experiencing decreased water pressure and delivery issues.
Lower pressure may impact homes in London, Essex, and Surrey, according to Affinity Water; King's Lynn has had supply outages, according to Anglian Water; and South East Water has also reported sporadic issues.
London, Manchester, and York are included in the Met Office's highest-level red heat advisory for Monday and Tuesday.
Since the introduction of the heat warning system last year, it has never been issued.
The amber alerts that were in effect for those days have been expanded to include all of Wales, southern Scotland, and the remainder of England.
An amber warning implies significant modifications are needed to work and habits, and health issues are more likely to affect some people.
Thomas Waite, the deputy chief medical officer for England, advised people to seek out cool locations and be well-hydrated during these rare few days.
Hospitals will be "very, really stretched" in the coming days, according to Lord Victor Adebowale, head of the NHS Confederation, who also told Times Radio that while the NHS "will cope," "coping isn't good enough."
Labour criticized the prime minister for allegedly not attending the emergency Cobra meeting on Saturday to discuss how to cope with the heatwave and for his intentions to hold a private lunch at Chequers for certain Conservative MPs on Sunday.
