A four-day "extreme heat" warning came into force in parts of England and Wales on Thursday, with temperatures poised to exceed 35 degrees Celsius (95 degrees Fahrenheit) in another scorching heat wave that could pressure water supplies and transport services.
The Met Office said its amber warning, its second-most severe after red, will last until the end of Sunday and cautioned that people vulnerable to extreme heat could face adverse health effects.
Temperatures are expected to peak at 35 degrees Celsius on Friday and may hit 36 degrees Celsius in some places on Saturday.
Much of England and Wales, plus eastern parts of Northern Ireland and Scotland, are forecast to be "sunny and hot or very hot" on Thursday, the Met Office's website showed.
The warning follows the driest July for England since 1935, when temperatures rose above 40 degrees Celsius for the first time, turning a renewed spotlight on the impacts of climate change.
Other European nations have also faced a scorching heat wave in recent weeks with temperatures often exceeding 40 degrees Celsius.
During July's heatwave, Britain, which is less used to such high temperatures, faced power outages, damage to airport runways and rail tracks and dozens of blazes in London, where the fire brigade faced its busiest week since World War II.
Britain's Environment Minister George Eustice on Wednesday urged water companies to take precautions to protect water supplies and tackle the effects of the prolonged dry weather.
Several water companies have already imposed restrictions on water usage and supermarkets have limited sales of disposable barbecues that firefighters warn can set light to tinder-dry grass.
This week's amber warning follows Britain's first-ever red "Extreme Heat" warning in July.
Source of Thames dries up
At the end of a dusty track in southwest England where the River Thames usually first emerges from the ground, there is currently scant sign of any moisture at all.
The driest start to a year in decades has shifted the source of this emblematic English river several miles downstream, leaving scorched earth and the occasional puddle where water once flowed.
It is a striking illustration of the parched conditions afflicting swathes of England, which have prompted a growing number of regional water restrictions and fears that an official drought will soon be declared.
The couple was planning to walk some of the Thames Path that stretches along its entire winding course – once they can find the waterway's new starting point.
"It's completely dried up," the IT worker from northern England told Agence France-Presse (AFP) in the village of Ashton Keynes, a few miles from the source, noting it had been replaced by "the odd puddle, the odd muddy bit."
"So hopefully downstream we'll find the Thames, but at the moment it's gone."
The river begins from an underground spring in this picturesque region at the foot of the Cotswolds hills, not far from Wales, before meandering for 215 miles (350 kilometers) to the North Sea. Along the way, it helps supply freshwater to millions of homes, including those in the British capital London.
Following months of minimal rainfall, including the driest July in England since the 1930s, the country's famously lush countryside has gone from shades of green to yellow.