The warning has been issued by the UK Health Security Agency (UKHSA) from 12pm on Monday until 6pm on Wednesday.
The alert covers Yorkshire and the Humber, the East Midlands, the East of England, London, the South East and the South West.
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The UKHSA said the alert means significant impacts are possible across health and social care services due to the high temperatures.
According to the Met Office, temperatures are set to hit 27C (81F) in parts of southern England over the weekend, with bright sunny weather in the early part of the week and temperatures expected to exceed 30C (86F) in places across parts of central, southern and eastern England on Monday and Tuesday.
The agency’s deputy chief meteorologist, Steven Keates, said: “We’re confident that temperatures will increase markedly by the start of next week, reaching the low 30s Celsius in parts of England on Monday and perhaps the mid 30s in a few places on Tuesday.
“However, the length of this warm spell is still uncertain, and it is possible that high temperatures could persist further into next week, particularly in the South.”
The hottest day of the year so far saw 34.7C (94F) recorded at St James’s Park in central London on 1 July.
It is officially a heatwave after three consecutive days at or above the threshold temperature, which varies between 25C for most of the UK and 28C (77 and 82F) for London, which is generally warmer.
Met Office spokesman Grahame Madge said: “We could be seeing parts of the UK have another heatwave by the middle of next week.
“I’d be surprised if areas didn’t make it on the duration and temperature value, but some places might reach those thresholds for a day or two before the temperatures come back down.”
Sky weather producer Kirsty McCabe said: “High pressure will extend from the Azores, allowing temperatures to rise, especially in the South where heatwave criteria is likely to be met early next week.
“The fourth heatwave of the summer will see temperatures reach the high 20s or low 30s, peaking around 33C (91F) on Tuesday across central and southern parts of England.
“Overnight temperatures will also rise, leading to some warm and stuffy nights from Monday night onwards.
“The current wet and unseasonably windy conditions across northern Scotland will also clear next week, with warmer weather expected here too.”
But she added the rising temperatures could spark off a few heavy and thundery downpours.
Heatwave conditions are forecast in parts of Europe, the Middle East, and North Africa, with temperatures touching the low-mid 40s in parts of Spain and Portugal, where overnight lows may be above 20C (68F).
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France may see higher than 40C (104F) on Sunday and Monday, while temperatures in the mid-30s are possible next week in the Netherlands and Germany.
She said there is a good chance of spotting the Northern Lights in the north of the country on Friday night, following a coronal mass ejection (CME) from the sun. Clouds in that area, however, may make it hard to view the spectacle.
