Parts of England have been sweltering in heat above 31C (88F), with near-record June temperatures forecast in Greater London and Surrey later
In Wimbledon, tennis fans have their strawberries and (sun)cream out on what looks to be the hottest opening day in history, while zoo animals in Leicestershire are being fed blocks of ice
Meanwhile, blistering heat is gripping parts of Europe, with "unprecedented" heat alerts in France and weather warnings in Spain, Portugal, Italy and Germany
In England, heat health alerts have been extended, but across the rest of the UK it's a mixed picture
Tell us how you're handling the heat on WhatsApp, external - and catch other readers' tips on staying cool
Watch: Today's weather forecast across the UK
We're interested in hearing from you to find out how the heat is affecting your day.
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Mark Poynting
Climate reporter
Forecasts suggest temperatures of 34C (93F) are possible on Monday.
These temperatures used to be very rare for the UK.
In the second half of the 20th Century, one in 10 years saw highs of 35C or more, BBC analysis of Met Office data shows.
But between 2015 and 2024, half of the years saw 35C or above.
As my BBC Weather colleague Simon King pointed out earlier, these temperatures are even more unusual for June - typically the coolest summer month.
It’s just one example of how very high temperatures are becoming more common.
Simon King
BBC weather presenter and meteorologist
In the last hour, the UK Health Security Agency (UKHSA) upgraded yellow heat-health alerts for Humber, Yorkshire and the West Midlands to the more severe amber.
This means significant impacts are likely across health and social care services due to the high temperatures, which are expected to reach 29C to 31C on Monday afternoon.
The North West is the only region in England to now have a yellow heat-health alert. North East England has no alert in force.
Temperatures overnight will mainly remain high, only falling to around 16C to 20C in the early hours of Tuesday morning.
While Tuesday could be hotter in London and South East England, it’ll turn a bit cooler on Tuesday for most people. But the amber heat-health alerts will remain in force until Wednesday 09:00 BST.
Sean Dilley
BBC News correspondent
Image source, Sean Dilley/ BBC NewsAs a guide dog handler of 26 years, my heart falls every time I hear the fatigued pants of dogs in heatwave weather.
Animal welfare charity the RSPCA says that dogs are 10 times more likely to suffer heat-related illnesses from exercising in hot weather than from overheating in cars. Both are tremendously bad for your dog.
Canines have a very limited ability to regulate their temperature. They don't sweat like humans - only perspiring a small amount through the pads on their paws.
Heavy panting, difficulty breathing, excessive drooling, lethargy and drowsiness are all key signs that your dog is too hot. In extreme cases, dogs vomit and finally lose consciousness.
If your dog is in trouble, the RSPCA says, external you should move the dog to a shaded and cool area and immediately pour cool, but not very cold, water over the dog, avoiding their head.
Watch: How to keep your pets cool in the heat
Image source, PETER JOLLYFires were reported in the Dava area of Scotland on Saturday night
It is inevitable that with the dry and hot days comes an increased risk of wildfires spreading.
Firefighters have spent a second night at the scene of wildfires north of Grantown-on-Spey in the Highlands.
The alarm was raised at 17:50 BST on Saturday in woodland near Loch Allan and spread over an area of several miles.
The Scottish Fire and Rescue Service says progress has been made controlling the blazes and adds that crews are monitoring the situation. Rain is forecast in parts of Scotland today.
Three roads remain closed as a result of the fires: the A939 from Ferness to Granton-on-Spey; the A940 from Dava to Logie; and the B9007 from Duthill to Ferness.
Gem O'Reilly
Reporting from Wimbledon for BBC Radio London
It's already 24C (75.2F) degrees here in SW19 and temperatures are expected to soar past 30C (86F) later.
Tennis fans who have joined Wimbledon's famous queue have set up umbrellas and deckchairs ahead of what could be a very long wait.
Image source, Gem O'Reilly / BBCDebbie and her son Reece have been queuing since 06:30 BST
The pair say they are ready for the hot weather and have been keeping cool using ice packs from their carefully-packed picnic.
Quote MessageWe've brought a picnic but we still want to get in and eat those strawberries and cream. It's brilliant to just be here, soaking up the sun and the atmosphere."
Tennis fan Debbie at Wimbledon
The UK Health Security Agency has extended the heat health alerts in place across England.
Amber alerts covering Yorkshire and the Humber, the East Midlands, West Midlands, East of England, London, South East and South West will now remain in place until 09:00 BST on Wednesday.
A yellow alert for the North West has also been extended to cover the same time frame.
There is currently no alert in place for the North East.
Health alerts serve to warn the public and provide guidance for NHS England, the government and healthcare professionals.
Mark Poynting
Climate reporter
It’s well-established that climate change is making heatwaves stronger and more likely.
As humans burn coal, oil and gas and cut down forests, carbon dioxide and other greenhouse gases are released into the atmosphere.
These gases act like a blanket, causing the planet to heat up.
Even a small increase in the Earth’s average temperature can shift heat extremes to much higher levels.
In parts of south-east England, where the highest temperatures are expected, a heatwave is declared when three consecutive days reach 28C or above.
Scientists at the World Weather Attribution group estimate that such conditions in June are now about 10 times more likely than before humans started to significantly warm the planet.
Adam Goldsmith
Live reporter
Image source, BBC Weather Watchers/George GroundhogGrey clouds gathering in Cumbria
I just stepped out from our newsroom in central London's Broadcasting House to grab a coffee and can confirm it's already sweltering in the capital.
We're hearing a much different story from our readers in the north of England, however.
"Heatwave, what heatwave?" writes Richard, who says it's "business as usual" where he is in west Cumbria.
Alec meanwhile tells us that where he's based, just outside Lancaster, "we are not suffering from the heat". Instead, he says it is currently "completely overcast and 19C".
Simon King
BBC weather presenter and meteorologist
Image source, Getty ImagesPeople in Lisbon shield themselves from the scorching sun
England isn’t the only place to currently be experiencing a heatwave.
It has been exceptionally hot across much of Europe over the last week.
Over the weekend, both Spain and Portugal provisionally recorded their hottest June days on record.
On Saturday, El Granado in Andalucía saw a temperature of 46C (114.8F) and on Sunday, in central Portugal, 46.6C (115.9F) was recorded in Mora, Évora.
Heat with temperatures above 40C (104F) is set to continue on Monday across Europe, with red heat warnings in Portugal, coastal Croatia and Sardinia.
We are enjoying seeing all of your pictures being sent in via BBC Weather Watchers.
Some of our favourites so far show pups being walked in the Midlands, flowers in bloom in Suffolk and Brighton looking resplendent under some clear blue skies.
Keep your pics coming!
Image source, Beefy / BBC Weather WatchersTwo dogs are getting in their steps early in Halesowen, Dudley, ahead of temperatures getting warmer later
Image source, Pogle / BBC Weather WatchersDeep blue skies have been pictured above the Royal Pavilion in Brighton
Image source, Juniperbeddy / BBC Weather WatchersOver in Lowestoft, Suffolk, a reader has sent in some images of her colourful flowers in full bloom
Image source, PA MediaRail travel is likely to be impacted throughout the day and various services have issued warnings for passengers to check the latest information before travelling and to carry water.
Currently National Rail has listed one weather-related disruption - affecting services between East Croydon and Burgess Hill from 10:00 BST - due to a speed restriction being implemented.
Meanwhile, Southern Rail has advised that trains will be busier than usual, particularly on routes to the coast, advising travellers to allocate additional time for their journeys.
Southeastern Railway has said coastal trains are expected to be busy between 10:30 and 13:30 and between 16:00 and 19:00 - asking passengers to try to avoid travelling during these periods.
The London to Brighton line has also warned of potential delays.
Image source, Getty ImagesWhile for many the warmer weather is generally a welcome change after months of dreary winter, it can be a bit of a nuisance and pose health risks for others.
Here are some reminders and pointers on how to stay cool and safe today:
It could be the hottest opening day in Wimbledon's history, according to BBC Breakfast's weather presenter Carol Kirkwood. But spirits are high among those who have been queuing overnight for centre court tickets at the tennis.
Vicky and Nigel from Swansea are number one and number two in the Wimbledon queue after camping overnight and say it has been "amazing, great fun".
Vicky has the number one ticket and she tells BBC Breakfast that Nigel and her did over argue over who would get it.
Meanwhile, a couple of friends who are numbers 284 and 285 in the queue say it was "boiling" waiting yesterday but thankfully was better in the evening once the sun went down.
"We're very excited; really looking forwards to it," one of them tells BBC Breakfast.
Tamsin Curnow
Reporting from Glastonbury
Image source, Andy Bennett / BBCIt’s a warm and muggy morning here after the final night of Glastonbury Festival.
There’s heavy cloud cover at the moment and it feels like a lot of people have heeded the festival’s advice to head off early to avoid fierce heat this afternoon.
A gentle stream of people has been heading out of the main pedestrian gate towards the bus station, where extra water taps have been set up to make sure everyone can get a cold drink.
And of course as they head off site, it’s what they leave behind.
The piles of recycling and rubbish are growing - notable spots so far include several air beds and a suitcase.
Image source, Tamsin Selbie / BBCSimon King
BBC weather presenter and meteorologist
Now we’re in summer, you might expect there to be spells of hotter weather, even heatwaves.
But it’s worth considering that 34°C (93.2F) or even 35°C (95F) is unusual for June and would make it one of the hottest June days on record.
These temperatures are around 12°C above the average.
Even widely across England and east Wales, temperatures on Monday will be about 8 - 10°C above where they would typically be.
As previously mentioned, it’s fresher across Scotland and Northern Ireland - with temperatures only slightly above average, especially in the east.
Image source, ReutersWhen Wimbledon opened its gates to last year’s tournament, those heading for the queue were advised to pack their umbrellas as rain threatened to delay play.
This year, the UK’s unpredictable weather once again looks set to play its part as heat health warnings loom.
Ahead of this year's tournament - which could see a record set for hottest opening day - a long line of campers spent a muggy night in tents in the park next door, where the traditional queue for tickets formed.
“Brits aren’t terribly good at knowing how to operate in really hot temperatures,” All England Lawn Tennis Club CEO Sally Bolton tells BBC Radio 4’s Today programme.
But, she says her teams are well prepared, advising tennis fans to "take time out of the sun", as well as making use of the more than one hundred water points once they finally arrive on site.
Image source, PA MediaFans queued in tents overnight with tickets for the event few and far between
Ben Rich
Lead Weather presenter
Image source, ReutersYou may have noticed that temperatures are often much higher in urban areas than in the surrounding countryside.
That is because of something called the urban heat island effect.
During heatwaves, city centres are on average 4-6°C (7.2-10.8°F) warmer than surrounding areas according to the European Commission Joint Research Centre, external.
In extreme cases the difference can be as much as 10°C (18°F) especially in very calm weather with little wind to mix the air.
A key reason for this contrast is the hard, dark-coloured surfaces, like buildings and roads, that dominate urban landscapes.
They absorb the Sun's energy rather than reflecting it, meaning they store heat. This is then slowly released into the surrounding air leading to higher temperatures, especially at night.
Waste heat generated by air conditioning units, vehicles - and humans - adds to this even further.
Image source, Getty ImagesFlames rise from a forest in the area of Izmir, Turkey
Temperatures across much of southern Europe are expected to remain above 40C (104F) today.
Health and wildfire warnings were issued over the weekend because of hot air from north Africa.
Two thirds of Portugal is on high alert; and there are similar conditions across Italy.
Elsewhere, an intense wildfire blazed across the Turkish tourist resort of Foça forcing hundreds to be evacuated from their homes.
Image source, Getty ImagesA woman buys a juice near Torre de Belem garden in Lisbon
Simon King
BBC weather presenter and meteorologist
For many, the heatwave will end after today. Cooler air will spread in from the north and west on Tuesday so temperatures will fall across northern, western and central parts of the UK.
The heat however will be squeezed into the far south-east of England where Tuesday could be even hotter. In Greater London, Surrey and Kent, there is a greater chance the maximum temperature somewhere reaches 35C (95F).
That will be the last day of the heatwave here. By Wednesday, all of the UK will be cooler at 17 to 24C.