A new June temperature record has been broken with the mercury staying above 23c overnight.
The Met Office confirmed the temperature only fell to 23.5c on Wednesday night at Bute Park, Cardiff.
A rare red warning for extreme heat has been issued by the Met Office as the UK continues to swelter in exceptionally hot and humid conditions.
The alert has now been extended to 9pm, Friday.
It is being driven by a "heat-dome" settling over western Europe that has brought extreme conditions across the continent, with human-driven climate change, mostly caused by burning fossil fuels, making heatwaves more frequent and intense.
The Met Office said it is possible the June record could be beaten again. If temperatures reach 39C, that would make it the second hottest day in the UK on record.
If 38C is recorded, that is still in the top five days ever recorded and hotter than any day of the 20th century.
Red heat health alerts have been issued by the UK Health Security Agency for the East of England, East Midlands, London, the South East, the South West and the West Midlands, and amber heat health alerts for the North East, North West, and Yorkshire and The Humber.
Britain is experiences record temperatures disrupting schools and transport networks in London,
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The Royal College of Physicians said doctors have described severe, unsustainable pressure on the workforce, with very few settings having air conditioning, staff "really struggling" and resident doctors battling the heat while sleep deprived.
One physician has warned "conditions are awful" due to overcrowding, another said two machines used to treat cancer had stopped amid the heatwave, and there had been reports of patients on geriatric wards, one of the groups most vulnerable to heat, facing temperatures of up to 35C.
Almost one in five employers have no measures to help workers cope with the heat, according to the Chartered Management Institute, with an uneven response to the heatwave.
Its survey of 1,000 managers found a third said their company had relaxed dress codes, while a quarter said flexible working hours were being offered, and a minority reported other measures such as free refreshments, additional breaks and reduced workloads.
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Met Office alerts are currently in place
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Disruption is expected across train networks in London and the South West with South Western Railway (SWR) extending its advice to passengers to avoid non-essential travel to include Friday.
SWR said: “Like on Wednesday and Thursday, SWR will operate a reduced service on Friday, with fewer trains running on its network. Trains that run will likely be busier, and journeys will take longer than usual.
“Services will also finish earlier in the day than normal, affecting return journeys.
"This includes trains and stations on the south coast, with customers strongly urged to carefully consider the advice to only travel if essential."
People have been encouraged not to travel to the beach
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Friday’s amber zone stretches from the South East to the North West and South Yorkshire. It does not cover Dorset and Hampshire’s coastal areas.
Steel rails can exceed 50C when the air temperature is 30C. Provisional figures suggest the temperature on Wednesday reached 36.1C in Gosport, Hampshire.
Deputy chief executive of Network Rail Jake Kelly said: "We know how important it is for people to get where they need to be, but this spell of very hot weather will make journeys more challenging across the network.
"Very high temperatures, like we are seeing in Great Britain and across parts of Europe, put significant strain on the railway, so our priority is to keep passengers safe while keeping services running.
"We’re asking passengers to think carefully about their plans and only travel if it’s absolutely necessary in the areas most affected by the extremely high temperatures that are being recorded."