While sun-worshippers are loving the surge in temperatures, millions could be at risk of heart and breathing problems due to a "toxic" combination of heat, pollen and pollution in the air.
In Edinburgh the mercury is set to soar this week as we experience some of the hottest days so far this year, while the Met Office predicts London temperatures will climb to a staggering 36°C.
This heatwave is reportedly the result of an "African plume" moving in from the south.
But it could prove deadly, putting the elderly and asthma sufferers - especially those with hay fever - at risk by exacerbating heart and breathing problems.
Dr Andy Whittamore, clinical lead at Asthma UK and a practising GP, said: "A toxic cocktail of hot humid weather and rising pollen levels this week could be extremely hazardous for the 5.4 million people in the UK with asthma, triggering deadly asthma attacks.
"Hot air and hay fever can cause people's airways to narrow, leaving them struggling to breathe, with symptoms like coughing, wheezing, a tight chest and breathlessness.
"Hot weather can also increase the amount of pollutants, pollen and mould in the air which can trigger asthma symptoms.
"If you are worried about the weather or hay fever affecting your asthma, make sure you take your hay fever medicines, keep taking your regular preventer as prescribed by your doctor and carry your blue reliever inhaler at all times."
Tips to stay safe include: staying out the sun at the hottest point of the day, shutting windows, drinking plenty of fluids and even checking on friends and relatives who are not as able.
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