Central Scotland, Tayside and Fife have now been included in the danger zone, with a hot spell building from Sunday and continuing over the next few days.
Forecasters are predicting that Scotland could this week see its hottest day ever recorded, breaking the previous official record of 32.9C – set at the village of Greycrook in the Scottish Borders in August 2003.
Members of the public are being urged to take care, while travel and services could face disruption.
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It comes as a more serious red warning was issued for the whole of Wales and parts of England for the first time, indicating a “danger to life”.
First Minister Nicola Sturgeon tweeted: ”The next couple of days are set to be exceptionally hot. Please take care and follow sensible advice - keep out of direct sun; stay well hydrated; and look out for others who may be suffering from the heat.”
Temperatures in some places could hit a record-smashing 40 degrees.