The rare Met Office alert covers a huge swathe of the country and will be force between midnight and 11.59pm on Sunday, July 17.
Some suggest the mercury could hit a record-breaking 43°C in places, with the possibility the warning could be extended into Monday.
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The weather warning reads: “Some exceptionally high temperatures are possible during Sunday and could lead to widespread impacts on people and infrastructure.
"Population-wide adverse health effects are likely to be experienced, not limited to those most vulnerable to extreme heat, leading to potential serious illness or danger to life.
"Government advice is that 999 services should be used in emergencies only; seek advice from 111 if you need non-emergency health advice.
“Substantial changes in working practices and daily routines likely to be required
“Significantly more people are likely to visit coastal areas, lakes and rivers leading to increased risk of water safety incidents
“Delays on roads and road closures are possible, along with delays and cancellations to rail and air travel, with potential for significant welfare issues for those who experience even moderate delays.”
The forecaster says temperatures will begin to build later this week, likely peaking on Sunday and Monday.
It added: “Some exceptionally high temperatures are possible and cumulative effects of warm nights and hot days are expected to bring widespread impacts to people and infrastructure.
“This period of hot weather is expected to continue into the early part of next week, hence an update extending this warning into Monday is likely.”