Britain experienced the hottest day of the year on Wednesday and temperatures are expected up to 39C (102.2F) in southern and eastern England, sparking widespread health and transport warnings.
The emergency services advised the public to take precautions in the heat and highlighted the risks of cooling off in the sea, lakes and rivers.
Several railway companies will run a significantly reduced service on Thursday with fewer trains and some could face speed restrictions as slow as 20mph.
National Rail said: “We strongly advise you to avoid traveling if you can.”
The Met Office said: “Temperatures are likely to be above average, perhaps more especially in the south and southeast where hot weather on the near continent may spread in at times…There is also an increasing risk of showers or thunderstorms”.
The TUC has urged employers to help workers cope with the extreme conditions by allowing flexible working to avoid the rush hour, keeping buildings cool and relaxing dress codes, wearing sunscreen and drinking plenty of water.
Dogs Trust, the animal welfare charity, has also urged owners not to leave their pets alone in hot cars as it can prove fatal.
Dr. Lynn Thomas, medical director at St John Ambulance, said “Heat exhaustion and heatstroke are two of the most serious problems that can develop when the mercury soars, but by being prepared you can spot the early warning signs such as headache and dizziness.
Owen Landeg, from Public Health England, said the summer heat brings “real health risks” for older people, young children and those with underlying health conditions.
“If you’re able, ask if your friends, family or neighbors need any support,” he said.
>Juthy Saha
