Temperatures could hit 40 degrees Celsius (104F), meaning a danger to life is likely, with the risk of illness not limited to vulnerable people.
The warning covers London and the Midlands and goes as far north as Manchester and York. Currently, an amber warning remains in Sussex. This amber warning is also in place for the whole country on Sunday (July 17).
A Met Office spokesperson said: “Exceptional, perhaps record-breaking, temperatures are likely on Monday, then again on Tuesday.
"Nights are also likely to be exceptionally warm for the UK, especially in urban areas.
"This is likely to lead to widespread impacts on people and infrastructure. Temperatures are expected to drop away from Wednesday onwards.”
The Met Office warned that will be an ‘exceptional hot spell’ on Monday and Tuesday, leading to ‘widespread impacts on people and infrastructure’.
People can expect to experience ‘population-wide adverse health effects’. The Met Office said these are ‘not limited to those most vulnerable to extreme heat, leading to serious illness or danger to life’.
A spokesperson added: “Substantial changes in working practices and daily routines will be required
“High risk of failure of heat-sensitive systems and equipment, potentially leading to localised loss of power and other essential services, such as water or mobile phone services
“Significantly more people visiting coastal areas, lakes and rivers, leading to an increased risk of water safety incidents
“Delays on roads and road closures, along with delays and cancellations to rail and air travel, with significant welfare issues for those who experience even moderate delays.”
The Government advice is that 999 services should be used in emergencies only. Seek advice from 111 if you need non-emergency health advice.
How we can stay cool
During the heatwave, the Met Office is advising people to; look out for others; close curtains to keep rooms cooler; drink plenty of fluids and avoid excess alcohol and dress appropriately.
People are asked to stay indoors where possible and parents are also being encouraged to limit their children's exposure to the sun.
Dr Agostinho Sousa, head of extreme events and health protection at UKHSA, said: “Heat-health alerts have now been issued to the majority of the country, with temperatures set to remain consistently high throughout the duration of this week.
“Most of us can enjoy the hot weather when it arrives, but it is important to keep yourself hydrated and to find shade where possible when UV rays are strongest, between 11am and 3pm.
“If you have vulnerable family, friends and neighbours, make sure they are aware of how they can keep themselves protected from the warm weather.’’