The Met Office has issued one of its new-style extreme heat weather warnings for the first time.
The amber warning covers large parts of Wales, all of south-west England and parts of southern and central England.
It will be in place until Thursday, when temperatures are expected to peak.
It warns of the potential impact of the heat on people’s health, as the UK experiences a prolonged period of sweltering conditions.
All four UK nations recorded the hottest day of the year over the weekend, and forecasters warned temperatures would continue to climb and could reach 33C (91.4F) in some western areas.