A national emergency has been declared, with a red warning for extreme heat issued for the first time for England on Monday and Tuesday - when temperatures could hit an unprecedented 40C.
Forecasters now say there is an 80% chance of the mercury topping the UK's record temperature of 38.7C (101.7F), set at Cambridge University Botanic Garden on 25 July 2019.
There is a 50% chance of temperatures hitting 40C (104F) somewhere in the UK, the Met Office said.
This is likely to be along the A1 corridor north of London up to Lincolnshire, in areas including Peterborough, Grantham, Sandy, and Stevenage.
The UK Health Security Agency (UKHSA) raised its heat health warning to Level 4 - a "national emergency" - on Friday.
Level 4 is reached when "a heatwave is so severe and/or prolonged that its effects extend outside the health and social care system".
This means fit and healthy people could be susceptible to illness and death - not just high risk groups, UKHSA said.
'Potentially very serious situation'
The first ever red warning issued by the Met Office means "adverse health effects" may be experienced and will not be limited to "those most vulnerable to extreme heat".
The "exceptional hot spell" will lead to "serious illness or danger to life" and bring "widespread impacts on people and infrastructure", the forecaster said.
"Population-wide adverse health effects" are expected, "not limited to those most vulnerable to extreme heat".
Met Office spokesman, Grahame Madge, described the rising heat as a "potentially a very serious situation".
Mr Madge said hitting 40C would be a "historic" moment.
"If we get to 40C, that's a very iconic threshold and shows that climate change is with us now," he said. "This is made much more likely because of climate change."
Elsewhere, the existing amber warning has been extended to the rest of England, Wales, and parts of Scotland from Monday.
England's chief medical officer, Chris Whitty, shared the signs of heat exhaustion on Twitter as he said "early intervention to cool people down and rehydrate them can be lifesaving".
Extreme heat 'absolutely unprecedented'
Met Office chief executive Penny Endersby described the extreme heat forecast as "absolutely unprecedented" and urged the public to take warnings seriously.
"Our lifestyles and our infrastructure are not adapted to what is coming," she said.
"Please treat the warnings we are putting out as seriously as you would a red or amber warning from us for wind or snow, and follow the advice."
Substantial changes to daily life required
"Substantial changes in working practices and daily routines will be required" to cope with the unusual conditions, the Met Office said.
The national weather service also warned there is a "high risk of failure of heat-sensitive systems and equipment, potentially leading to localised loss of power and essential services, such as water or mobile phone services".
There will also be "significantly more people visiting coastal areas, lakes and rivers, leading to an increased risk of water safety incidents".
Senior ministers assembled for a COBRA meeting earlier this week in anticipation of a national heatwave emergency being declared.
Cabinet Office minister Kit Malthouse warned the government is preparing for a surge in demand on the NHS and other services - as he urged the public to look out for people vulnerable in the heat.
Transport delays anticipated amid fears roads could melt
The public has also been warned to expect delays on roads, along with disruption to rail and air travel.
More than 1,000 extra cars are expected to break down, the RAC said - calling on people to check their oil and coolant are at the right levels and ensure tyres are damage-free and inflated correctly.
Drivers are advised to carry emergency supplies including water, food, medication, a sun-blocker - such as an umbrella - and a fully-charged mobile phone.
Those without effective air-conditioning should consider delaying non-essential journeys.
Public transport is also set to be affected due to the impact of the heat on train lines.
Network Rail said slower speeds may need to be implemented and reminded people to carry water on journeys - and only travel if necessary.
Services could be cancelled at short notice, and on Monday and Tuesday will be subject to revised timetables.
South Western, East Midlands Railway and TransPennine Express are among those that have already warned passengers their trains will be disrupted.
The Department for Transport is also in discussions with port operators, highways agencies and the police to make plans in particular for places where queues are likely to build up.
Schools take precautions to protect pupils
Schools in some areas are allowing children to wear PE kits or loose, light-coloured clothing in a bid to combat the heat.
Others will allow pupils to start and finish early to avoid the hottest part of the day, with some sports days also rescheduled.
But there's no blanket ruling from central government that schools should close on Monday and Tuesday.
The Department for Education said "individual school leaders are responsible for managing their own local circumstances".
Sales of paddling pools, fans, and ice cream soar
Retailers say the scorching temperatures have led to a record-breaking amount of ice creams and lollies sold in a single week.
Tesco is expecting to sell more than nine million frozen treats this week, while Waitrose said ice cream sales had increased by 20% compared to last week.
Demand for sun protection products rocketed by 220%, the supermarket said.
Meanwhile, John Lewis has reported fan sales were up 256% compared to last year, while blackout curtains were up 193%.