The UK Health Security Agency has issued a heat-health alert for southern and central England from Tuesday to Saturday.
Experts advise people to look out for young children and people who are older or with existing health conditions.
Much of the UK is braced for another heatwave, with little rain expected to help relieve the threat of drought yjay has led to hosepipe bans and fire warnings.
The Met Office said temperatures were likely to rise into the low to mid-30s in central and southern parts of the UK, but will not be as extreme as the record-breaking heat in July when the mercury climbed above 40ºC.
Heatwave thresholds, which are met at different temperatures in different parts of the country, are likely to be hit in much of the UK.
Outside the hottest areas, much of England and Wales and south-east Scotland could see temperatures widely in the high 20s, with a chance of a few spots moving into the low 30s, the Met Office said.
Scotland and Northern Ireland will also see temperatures in the high 20s and could reach official heatwave criteria by Friday, forecasters said.
With the latest heatwave coming after months of low rain, which have left the countryside and urban parks and gardens tinder-dry, households in some areas are being urged not to light fires or have barbecues.
Essex County Fire and Rescue Service is urging people not to light barbecues or bonfires, or let off fireworks or sky lanterns, after a large blaze that damaged gardens, sheds and trees was started by a chiminea.
The Country Land and Business Association, which represents 28,000 farmers and landowners in England and Wales, has demanded retailers follow the lead of Marks & Spencer and ban the sale of disposable barbecues across the UK this summer to reduce the risk of wildfires in the dry conditions.
Heatwave in the UK – in pictures
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People basking in the sun on a crowded Brighton beach. The temperature in Britain smashed through the 40ºC barrier for the first time in recorded history. PA -
A man dives into the Serpentine Lake to cool off in Hyde Park, west London. AFP -
Firefighters rest as they attend a gorse bush fire, near Zennor, Cornwall. Reuters -
A tourist wears a hat to shield herself from the sun on Westminster Bridge in central London. AP -
A man cools off at Trafalgar Square, central London. Reuters -
Firefighters respond to a large wildfire in woodland at Lickey Hills Country Park on the edge of Birmingham. PA -
A railway worker hands out bottles of water to passengers at London's King's Cross, where there are train cancellations due to the heat. AP -
Firefighters control a grass fire in Ravenswood, Suffolk. PA -
Train tracks are painted white to help with the heat at Alexandra Palace train station in London. PA -
Deer rest in the shade at London's Richmond Park. Reuters -
An empty freezer section at Sainsbury's Nine Elms in London. PA -
Chippy the chimpanzee enjoys an ice treat at Blair Drummond Safari and Adventure Park, near Stirling. AP -
A student dips her head into the fountain at Trafalgar Square, London. Getty Images -
People turn out to watch the sunrise on Tuesday morning at Cullercoats Bay, North Tyneside. PA -
A digital thermometer displaying a temperature of 39°C on a London Underground train. PA -
A fan finds a way to keep cool while watching the Women's Euro 2022 football match between Italy and Belgium at Manchester City Academy Stadium. AP -
An aerial view shows swimmers at Hathersage Swimming Pool, west of Sheffield in northern England. AFP -
Traffic crosses a bridge at Woodhead Resevoir in West Yorkshire as water levels dip dangerously low. AP -
A man leaps from the top of a tree into the River Cam, in Cambridge. Getty Images -
A packed Gyllyngvase beach in Falmouth. Getty Images -
A police officer gives water to a sentry in heavy ceremonial uniform, outside Buckingham Palace in central London. AP -
A man cools off in a fountain in London. Reuters -
A man sunbathes at the harbour in Mousehole, Cornwall. AP -
People swim in the River Wye. Reuters -
Passengers feel the heat on the London Underground. The operator advised its customers not to use the metro during the worst of the heatwave. AP -
Palm House supervisor Will Spolestra waters the plants at the Royal Botanical Gardens Kew, west London, where temperatures inside the greenhouses are cooler than outside during the heatwave. PA -
Dry grass at Greenwich Park, south-east London. AP -
A swimmer takes an early morning dip in the cool water of Jesus Green Lido in Cambridge, eastern England. Getty Images -
Assistant horticulturalist Katie Martyr checks the temperature at the Cambridge University Botanic Garden. Forecasts are for record temperatures exceeding 40°C. Getty Images -
Bathers keep cool in a tidal pool at Perranporth Beach in Cornwall, south-west England. Reuters -
Message boards at London's Victoria Station warn passengers of potential disruption to train services due to the extreme heat. AFP -
Zorro the police horse drinks water from a bucket to cool down during hot weather, on Whitehall in London. Reuters -
People use a map to shelter from the sun outside Buckingham Palace in London. Reuters -
A cyclist rides through Richmond Park in south-west London at sunrise on Monday. Reuters -
The reduced water level at Weir Wood reservoir, near Crawley, south-east England. The UK's Met Office has issued its first 'red warning' for exceptional heat. AFP -
A crowded beach at Joss Bay in Broadstairs, south-east England. Reuters -
Swimmers take the plunge to escape the heat in east London. Reuters -
Sunbathers on the parched grass in Greenwich Park, south-east London. AP -
A sign advises people not to travel on the London Underground during the heatwave. EPA
The Met Office's fire severity index, an assessment of how severe a fire could become if it were to start, is very high for most of England and Wales, and will reach "exceptional" for a large part of England by the weekend.
Two water companies have already announced hosepipe bans and others have warned they may need to follow, after the driest eight months from November to June since 1976, and the driest July on record for parts of southern and eastern England.
Scientists warn that the likelihood of droughts occurring is becoming higher due to climate change, driven by greenhouse gas emissions from burning fossil fuels and other human activities.
Climate change is also making heatwaves more intense, frequent and likely, with last month's record temperatures made at least 10 times more likely because of global warming, research shows.
Fires break out in UK during heatwave - in pictures
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A residential area after a large fire in Wennington, Greater London. Several fires broke out across England as the UK experienced a record-breaking heatwave. Getty Images -
Residents look at buildings destroyed by fire in Wennington. Getty -
Firefighters tackle a grass fire during the heatwave in Mow Cop, Staffordshire. Reuters -
Firefighters in Maltby, after a fire started on scrubland before spreading to outbuildings, fences and homes in South Yorkshire. PA -
The scene after a fire at Dartford Marshes. PA -
Burnt woodland is seen as smoke rises from the trees following a fire in Blidworth. Getty Images -
A car drives towards a fire in East London. Reuters -
Burnt woodland in Dartford Heath, England, after a fire on Tuesday. Getty -
Smoke columns rise from Dartford, Kent, where a fire erupted earlier in the day. AFP -
Smoke rises from Wennington neighbourhood fires in England on Tuesday. Getty -
Emergency services tackle a fire on Dartford Heath. Grass fires broke out around the country during an intense heatwave. Getty -
People look on as a fire burns in East London. Reuters -
A firefighter attends a gorse bush fire during a heatwave near Zennor, Cornwall. Reuters -
Smoke pours from a blaze in the village of Wennington, east London. PA -
Firefighters hose down a grass fire in Ravenswood in Suffolk. PA -
Firefighters rest after attending a gorse bush fire near Zennor in Cornwall. Reuters -
A large wildfire in woodland at Lickey Hills Country Park on the edge of Birmingham. AP -
Smoke from the fire near Zennor, Cornwall. Reuters -
Burnt woodland alongside a motorway near Dartford Heath. Getty -
A firefighter has a drink of water after the major blaze on Dartford Heath. Getty
The Met Office recently raised the temperatures that have to be reached for an official heatwave for eight English counties, to reflect the warming conditions in the UK.
"Heatwave criteria look likely to be met for large areas of the UK later this week, with the hottest areas expected in central and southern England and Wales on Friday and Saturday," said deputy chief meteorologist Tony Wardle.
"Temperatures could peak at 35ºC, or even an isolated 36ºC on Saturday.
"Elsewhere will see temperatures widely into the high 20s and low 30s Celsius later this week as temperatures build day on day through the week due to an area of high pressure extending over much of the UK," Mr Wardle said.
"Coupled with the high daytime temperatures will be continued warm nights, with the mercury expected to drop to only around low 20s Celsius for some areas in the south."
The Met Office said there is little rain in the forecast, with only the north-west likely to see any short-lived showers.
"Further south, which has seen little rain for some time now, dryness will continue through the week and provide no relief for parched land, especially in the South East," Mr Wardle said.
Richard Allan, professor of climate science at the University of Reading, said there were many reasons why drought events become worse as a result of human-driven climate change.
A warmer atmosphere is thirstier and dries out the ground, while heatwaves exacerbate the development of drought conditions, and, because continents are warming so fast, ocean winds cannot blow enough moisture over the land.
Uneven global warming can also disrupt weather patterns, and make periods of more persistent wet or dry conditions more common.
"Human caused warming of climate is intensifying the global water cycle and disrupting weather patterns leading to more severe droughts but also more serious flooding events across the globe," said Prof Allan.
UK heatwave sees highest-ever recorded temperatures - video
Dr Leslie Mabon, lecturer in environmental systems at The Open University, said: "Above all else, the drought risk we are seeing in the UK is a reminder that we urgently need to tackle the problem at source.
"This means reducing emissions from fossil fuels to limit the extent of harmful climate change we will face.
"Moreover, countries like the UK, which have traditionally had more a more temperate climate and have less experience of managing the prolonged effects of hot, dry spells, need to plan now to adapt to hotter weather.
"More than encouraging individuals to save water, this also means looking at our water infrastructure and considering where investments are made to ensure we are better prepared for managing water in hot spells."
