The unusually hot weather across England has claimed the lives of hundreds people in just nine days, scientists have said.
Researchers estimated that the heatwave has lead to 650 deaths in England between July 6 and 14 as the soaring temperatures show no sign of abating.
Ben Armstrong, professor in epidemiological statistics at the London School of Hygiene & Tropical Medicine, said that the hot weather could have led to the premature deaths of between 540 and 760 people.
The news came as health officials advised people to stay cool, drink lots of cold fluids and keep an eye on those they know to be at risk, and after MPs called for workers to be sent home if temperatures exceed 30 degrees Celsius.
Professor Virginia Murray, head of extreme events and health protection at Public Health England (PHE), told The Times: "The risk of death and risk of illness really concerns us.
"Those with pre-existing illnesses are at much greater risk of not being able to cope with heat. It's much harder for them to cope with cooling."
She told the newspaper that people with breathing difficulties could find themselves struggling to get enough air to regulate their temperature. And those with heart difficulties are also more likely to suffer heart attacks, she said.
Dr Angie Bone, heatwave plan leader for PHE, said: "In this continued hot weather, it's important to remember that high temperatures can be dangerous, especially for people who may be vulnerable such as older people, young children and those with serious illnesses.
"During very hot weather, pregnant women and people who have chronic illnesses, including cardiovascular, respiratory, renal conditions, diabetes or Parkinson's disease, may experience discomfort if indoor temperatures are particularly hot and in using public transport."
Prof Armstrong created the mortality statistics for the newspaper by using temperatures supplied by the Met Office and comparing it with studies he has conducted during previous heatwaves in Britain.
The news comes as the Met Office has raised heatwave alerts in more parts of Britain.
South West England and the West Midlands have been elevated from level two to level three, putting the regions on a par with the South East and London.
The Met Office said that there is a 90% probability of heatwave conditions between 9am today and 9pm tomorrow in parts of England.
The country experienced the hottest day of the year so far yesterday, with the mercury hitting 32.2C (90F).
Temperatures are expected to hit 30C (86F) again today, according to MeteoGroup, the weather division of the Press Association.
The East Midlands, Yorkshire and the Humber, and the East of England remain at level two. The North East and North West remain at level one, the Met Office said.
Forecasters have said Britain is in the midst of its first prolonged heatwave since 2006, with six consecutive days of temperatures above 30C (86F).
It is set to continue into next week with temperatures in the high 20s Celsius at the weekend before rising to 30C and above next week - bringing with it an increased risk of thunderstorms.
Gemma Plumb, a forecaster with MeteoGroup, said there was no sign of temperatures dropping significantly in the coming days.
"On Saturday and Sunday it looks like we will have temperatures of around 28C (82.4F) but by Monday we have temperatures of 29C and 30C, with an increased risk of getting some showers," she said.
"Today there is a risk of thunderstorms across parts of south-east Scotland, South Wales and south-west England, but the rest of the country should remain dry."
Temperatures reached 32.2C (90F) at Hampton in Surrey yesterday, making it the warmest weather since Saturday.
MeteoGroup said average rainfall for England and Wales so far this month was only 15% of the average at 4.9mm, while Thames Water said it had seen a 15% rise in the demand for tap water in London and the Thames Valley.
The Met Office said the UK had seen 132 hours of sunshine between July 1 and July 15, which is 77% of the average for the whole month.
Average daytime temperatures have been 2C above average, it said.
Police and fire chiefs yesterday reiterated warnings about escaping the heat by swimming in open water after four people died in separate incidents on Tuesday.
A 16-year-old boy and a 41-year-old man, from King's Lynn, were found dead in unrelated incidents in lakes at Bawsey Pits, Norfolk.
A man, believed to be a 21-year-old from Shropshire, died after getting into difficulties near the confluence of the Dee and Ceiriog rivers on the Shropshire-North Wales border.
A 40-year-old woman from Saltash died following what police called a "medical episode" while swimming in the sea at Seaton, near Torpoint in eastern Cornwall.
Reports of dogs being left in cars with windows closed in the sweltering conditions have prompted warnings from the police.
Kent Police received five reports of dogs left in cars in the hot temperatures on Tuesday. Police said leaving a window open is not enough to prevent a dog from dying in the heat.
In one incident, firefighters and the RSPCA rescued a dog in a distressed state after it was left in a car with a window partially open at a shopping centre in Ashford.
Pc Michael Laidlow said: "Dogs can die in hot cars and should not be left for even a short time. Leaving a window open won't keep the car cool enough.
"Dog owners are reminded that they have a legal duty of care for their animal and can be prosecuted for putting their pets at risk."
Prof Armstrong said: "Our previous studies have shown that as temperatures rise above a certain threshold, the risk of death increases.
"Using the same model, we estimate that the current heatwave has caused the premature deaths of 650 people. The excess is likely to have been overwhelmingly among the elderly, especially those over 75, some of which may have been among people who would have died just a few weeks later if there had been no heatwave."