The British Meteorological Institute sent out on Monday the notice for South Wales, the West Midlands and the south and south-west of England.
Large parts of the UK have had heatwave conditions already in recent days, but it is expected that the temperature will rise further at the beginning of this week.
Some areas can get temperatures up to 33 degrees.
The new level for weather forecast will be until Thursday, when meteorologists expect that the heat will decrease, writes Sky News.
Met Office created the new the extreme heat forecast in June this year, and this will be the first time it is used.
The reason the possible health consequences of persistently high temperatures and heat wave conditions.
The risk that the heat can affect infrastructure, business and traffic, as well as the increased risk of forest fires, are also reasons why the Met Office now warns of extreme heat.
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