Heatwave alerts
The Met Office issues severe heatwave alerts across the country, when temperatures are expected to exceed 30C (86F) in some areas.
A level three heatwave alert means high daytime temperatures and warm and humid nights are forecast.
Public Health England urged people to be aware of the health risks of heat.
To stay safe, people should drink plenty of water, wear loose-fitting cotton clothing, and stay out of the sun between 11:00 and 15:00 BST, it said.
Heatwave myths and how to stay cool
How hot is it where you are?
Heatstroke – the heatwave killer
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What is a heatwave?
Although there is no official definition of a “heatwave” in the UK, the term can be used to describe an extended period of hot weather relative to the expected conditions in an area at that time of year.
This is different to the Met Office’s four heatwave alert levels, which trigger action from health professionals and emergency planners.
The criteria vary by region – for example, the daytime temperature threshold in north-east England is 28C, but in London it is 32C. The average threshold temperature is 30C by day and 15C overnight for at least two consecutive days.
Parts of the UK are currently at level three of four – four being a “national emergency” red alert, which is issued when a prolonged hot spell becomes severe.
Source: Met Office
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Public Health England (PHE) also urged people to look out for older people, those with underlying health conditions and young children who may be particularly vulnerable as temperatures rise.
It also advised people to:
Close curtains on rooms that face the sun to keep indoor spaces cooler – but remember it may be cooler outdoors than indoors
Drink plenty of water – sugary, alcoholic and caffeinated drinks can make you more dehydrated
Open windows when it feels cooler outside
Never leave anyone in a closed, parked vehicle – especially young children or animals
Walk in the shade, apply sunscreen and wear a hat when outside
Avoid physical exertion in the hottest parts of the day
People should also take care and follow local safety advice if going swimming in open water to cool down, PHE added.
Six people died around the UK and Channel Islands coastline in a series of incidents last weekend.
How to stay safe at the beach
Dr Thom Waite, a consultant in public health medicine at PHE, said: “Each year we hear stories of people who have fallen seriously ill because, even though it’s hotter, they may wear clothes which are too warm for hot weather, they may not drink enough or try to do too much.”
He said as it was still the school holidays for many, and with a number of high-profile events planned for this weekend, people should “stay hydrated, look out for others and keep an eye on forecasts”.
Wales and north England are also expected to experience warm temperatures this week, reaching highs of about 28C (78.8F) on Tuesday afternoon.
Met Office meteorologist Steve Willington said the warm and sunny conditions were expected to hold in the south-east of England until Friday.
However, Tuesday’s high temperatures could give way to scattered thunderstorms and showers pushing up from the south-west across the West Midlands towards Humberside from Wednesday evening and into Thursday.
Scotland and Northern Ireland are forecast to experience lower temperatures throughout the week, with cloud and some showers possible.
More unsettled weather is expected for the bank holiday weekend – although the Met Office said there was a chance for a brief return to warmer conditions early in the weekend across at least southern parts of the UK.
Heatstroke – the heatwave killer
Temperatures are rising across Europe and while the sun is out so are the health warnings.
Heatwaves can have a profound effect on the body beyond sunburn – it can be deadly.
The earliest, and relatively mild, heat-related issues are:
heat cramps caused by dehydration (it’s often linked to exercise)
heat rash
heat oedema which mainly takes the form of swollen ankles
The more serious problems include heat exhaustion, which develops into fatal heatstroke.
Sick and faint
Sweating is the main method your body uses to keep cool as temperatures rise.
But all the time you’re sweating you’re also losing salts and water and it leads to a drop in blood pressure.
Symptoms of heat exhaustion are similar to being dehydrated – feeling sick, faint and sometimes shock.
Treatment should be relatively easy at this stage – drinking plenty of water and moving to a cooler place.
But things can become much worse and develop into heatstroke.
Eventually the body’s ability to sweat shuts down as fluid levels become dangerously low.
Someone who once looked bright pink and sweaty becomes pale with hot dry skin.
With no way of cooling down, core body temperature can soar above 40C (it is normally 37C).
Symptoms at this stage include confusion, disorientation, convulsions and falling into a coma and can culminate in organ failure, brain damage and death.
Suspected heatstroke should always be considered a medical emergency.
Thunderstorms
The heat can also exacerbate a wide range of other health conditions.
The extra strain on the heart because of changes in blood pressure can increase the risk of heart attacks and stroke.
There is even “thunderstorm asthma” as the number of attacks increases after some thunderstorms, although the reason is not entirely clear.
The point at which heat becomes a problem varies widely, as there are differences between people and countries.
People who struggle to control their temperatures – including babies, the elderly and those on some medications – will be affected at lower temperatures.
Dr Angie Bone, the head of extreme events and health protection at Public Health England, says there are also big differences between countries.
She told the BBC: “If you look at temperature and mortality – even within England – the threshold we start to see increases in deaths is different in the south and north of the country.
If you’re regularly exposed to these temperatures then your body acclimatises.
But also how countries organise buildings and cities has an effect – compare ours with the Mediterranean. They have more shade, external shutters on buildings and air conditioning as they’re used to these temperatures.”