The UK Health Security Agency (UKHSA) alerts cover East Midlands, East England, London and the South East from Thursday to Saturday.
It warns that expected hot weather may have minor impacts on the health and social care sector.
The Met Office said temperatures of “around 30C are possible in the South East” and that figures in the mid to upper twenties are “more likely in places” across England and Wales.
Here is the hour-by-hour for today (Thursday)
10am – Sunny intervals 20C
11am – Sunny intervals 20C
12pm – Sunny intervals 22C
1pm – Sunny intervals 23C
2pm – Sunny intervals 24C
3pm – Sunny intervals 26C
4pm – Sunny intervals 26C
5pm – Sunny intervals 27C
6pm – Sunny intervals 26C
7pm – Sunny intervals 26C
8pm – Sunny intervals 25C
9pm – Cloudy 24C
10pm – Cloudy 23C
Here is the hour-by-hour forecast for tomorrow (Friday)
7am – Sunny intervals 19C
8am – Sunny intervals 21C
9am – Sunny intervals 22C
10am – Sunny intervals 24C
11am – Sunny 26C
12pm – Sunny 28C
1pm – Sunny intervals 29C
2pm – Sunny 30C
3pm – Sunny 31C
4pm – Sunny 31C
5pm – Sunny 31C
6pm – Sunny 30C
7pm – Sunny 29C
8pm – Sunny 28C
9pm – Clear 26C
10pm – Clear 24C
Forecast in full
Today: Any fog patches clearing quickly to leave a fine day with plenty of sunshine, perhaps hazy at times during the morning. Warm or very warm with light winds. Maximum temperature 28 °C.
Tonight: Plenty of evening sunny spells with clear spells overnight. Chance of some isolated showers moving northeast from the south coast. Remaining warm or very warm. Minimum temperature 18 °C.
Friday: Dry with plenty of sunshine, turning hazy at times mainly during the morning. Hot with light winds. Maximum temperature 30 °C.
Outlook for Saturday to Monday: Another warm day on Saturday with the risk of heavy showers and thunder. Sunny spells with scattered showers and feeling fresher on Sunday. Cloudy and breezy with rain on Monday.
Met Office meteorologist Alex Deakin said Wednesday has been “a fine day for many” which is to be followed by a “hot spell for many coming up.”
In an online forecast he said: “Further south and east, after some early morning mist and fog perhaps, we will see plenty of sunshine and that’s going to lift the temperatures widely in to the mid-20Cs and 28C is likely in and around the London area.
“It will be quite a bit cooler for Scotland and Northern Ireland where temperatures here may be 18C or 19C or maybe a bit lower where that rain persists on the west coast.”
Cooler conditions are set to be in place by Sunday but “certainly there will be some high temperatures across eastern England during Friday and Saturday”, he added.
The hot weather comes after well above average rainfall figures in the first half of the month, including a weather warning for rain on Tuesday evening.