The amber warning covers large parts of Wales, all of south-west England and parts of southern and central England and will be in place until Thursday, when temperatures are expected to peak.
The Met Office launched its new extreme heat warning in June 2021 to highlight potential widespread disruption and adverse health effects.
The warning came after all four UK nations recorded the hottest day of the year over the weekend.
Rob Matthews, of rural insurance broker Lycetts, urged farms – particularly mixed farms – to not overlook the effect of rising temperatures on livestock and to take precautionary measures against heat stress.
He said: “Farming is a weather dependent industry, acutely sensitive to temperature extremes, so the summer can be an extremely worrying and testing time of year for farmers, particularly as the weather increasingly oscillates between heavy rainfall and soaring temperatures due to climate change.
“Harvest often takes precedence in these critical months, with time and effort concentrated on maximising yields.
“Of course, the wellbeing of the livestock is also top priority for farmers, but it takes just a few extra degrees, over a relatively short period of time, for animals to suffer the effects of heat stress – it can happen quickly and without much warning.
“Unfortunately, many animals do not have the ability to dissipate enough heat to maintain homeothermy, so even seemingly subtle increases in air temperature can be very harmful and can lead to death.
“An increase in body temperature can also impact on milk productivity in dairy cows, and cause beef cattle to lose condition.
“Farmers care deeply about the wellbeing of their animals and losing animals in this way can be very distressing.”