Temperatures are forecast to rise over the coming days, reaching a sweltering 32C on Tuesday.
The Met Office's amber weather warning was in place for Sunday but had now been extended to include all of Monday and Tuesday too.
It says: "A hot spell is likely to develop from Sunday, likely peaking early next week, leading to widespread impacts on people and infrastructure.
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"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."
It adds: "Temperatures will rise again this weekend, most likely peaking on Monday or Tuesday, then most probably declining thereafter.
"Latest evidence supports the idea of a trend towards a slightly later onset of high temperatures. Some exceptionally high temperatures are possible both by day and by night; the cumulative effects of very warm nights (particularly in urbanised areas) and hot days are likely to bring widespread impacts to people and infrastructure."