BBC Breakfast reporter Ben Boulos has been talking to pig farmers at the Euston Estate in Suffolk to find out how they are coping with the dry weather.
East Anglia has had 67% of its normal rainfall so far this year - 185mm (7.2in) - making it the driest January-to-June period since 1996.
The River Waveney between Norfolk and Suffolk is running at about 30% of its normal flow.
Chris Fogden, from Euston Farms, says: "The pigs get quite stressed in this weather so we do everything possible to let them keep cool [such as having] insulated huts, plenty of water, and not handle them unnecessarily.
"We've got a very good private borehole system so we're not reliant on mains water and we supplement what comes out of the pipe with a big tanker."
North Norfolk District CouncilCopyright: North Norfolk District Council
Smoking has been banned on Cromer Pier while the hot weather continues.
In a Facebook post, North Norfolk District Council says: "During this extended period of dry, hot weather there will be no smoking permitted on Cromer Pier."The timber boards, when warm and dry, are a fire risk from the embers and discarded cigarette butts."
It beat the previous UK record of 38.5C (101.3F), set in Kent in 2003.
A Met Office official checked the equipment in central Cambridge and verified the new record.
CUBGCopyright: CUBG
This is the weather station at Cambridge University Botanic Garden where the hottest temperature so far in the UK was recorded in 2019Image caption: This is the weather station at Cambridge University Botanic Garden where the hottest temperature so far in the UK was recorded in 2019
At the time, staff at the garden tweeted to say they felt like they had "melted".
Northamptonshire - North Northamptonshire and
West Northamptonshire councils say it is the responsibility of the school or
academy to notify of a closure and has a schools directory on its website to
find the contact details.
Milton Keynes Hospital: Outpatient appointments and surgery cancelled
Routine outpatient appointments and surgery will not go ahead at Milton Keynes University Hospital on Monday and Tuesday following the warning over extreme heat.
Milton Keynes University HospitalCopyright: Milton Keynes University Hospital
Chief executive Joe Harrison says the NHS trust took the decision because many patients who would be travelling to these appointments are "frail and at increased risk" and the "unpredictable nature of very high temperatures" would affect "demand for emergency care and the care environment".
"We will do all we can to keep wards and departments as cool as possible, but we know this will be a challenge given the very high outside temperature," he said.
What does the red weather warning mean?
A red extreme heat warning has been issued by the Met Office as temperatures could hit 40C (104F).
It is the first time a red heat warning has been issued for parts of the UK, although the extreme heat warning system was only introduced in 2021.
BBCCopyright: BBC
This is what the Met Office said people should expect:
Adverse health effects, not limited to those most vulnerable to extreme heat, leading to serious illness or danger to life. Government advice is that 999 services should be used in emergencies only and people should seek advice from 111 if they need non-emergency health advice
Substantial changes in working practices and daily routines will be required
A high risk of failure of heat-sensitive systems and equipment, potentially leading to localised loss of power and other essential services, such as water or mobile phone services
Significantly more people visiting coastal areas, lakes and rivers, leading to an increased risk of water safety incidents
Delays on roads and road closures, along with delays and cancellations to rail and air travel
Both carriageways were shut after the crash between junction 16 for Northampton and junction 18 for Daventry, which happened at about 03:30.
The force says the road is "expected to remain closed throughout the morning" and diversions are in place.
In a tweet, it said: "Due to the extreme temperatures expected in the county today, we would ask all drivers to avoid the area and to seek an alternative route."
Getty ImagesCopyright: Getty Images
Prof highlights climate change in hot-spot of Cambridge
BBC Breakfast on BBC One has been live in Cambridge this morning.
Prof Emily Shuckburgh, director of Cambridge Zero at the University of Cambridge, says the country is not geared up for a Mediterranean climate and we need to tackle climate change.
"The biggest thing we could be doing is to reduce our emissions in the first place, because that is how we stop climate change," she says.
UK heat: Monday and Tuesday compared to the last 60 years
The year 1976 ranks as having the second-warmest summer in England on record, however the highest temperature recorded then was 35.9C (96.62F).
Dan Holley from Weatherquest said these maps highlight the difference in extent and intensity of the heat between 1976 and the highest temperature recorded in the past 60 years - compared to forecasts for today and tomorrow.
Dan HolleyCopyright: Dan Holley
He said: "We are expecting some incredibly high temperatures here in the East, potentially as high as 40C (104F) on Monday and Tuesday.
"On top of that the nights will be very warm as well, some places may not get below 20C (68F)."
The hot summer of 1976 saw water-rationing and it's the go-to year when talking about heatwaves for most people. Here are some BBC features about it:
Whipsnade Zoo said it would be open on Monday and Tuesday, but said it may need to close "some indoor areas", some animals may not be on show and some talks may be cancelled.
It advises people who are visiting to "please drink plenty of water and wear sunscreen".
ZSL Whipsnade ZooCopyright: ZSL Whipsnade Zoo
In a Twitter post, it says: "The welfare of our animals, colleagues and visitors is paramount.
"Our animals have plenty of choice about where they go and what they do and many will have 'cool zones' to retreat to which aren't visible to visitors.
"Some exhibits may be closed to manage the temperature inside."
Good morning
A red extreme heat warning has been issued by the Met Office for Monday and Tuesday - as temperatures are forecast to hit up to 40C (104F).
As temperatures rise, we'll be here to show you how the East of England is being affected.
People have been up very early this morning to take their dogs out while the ground is still relatively cool - such as in St Albans in Hertfordshire (below).
Live Reporting
All times stated are UK
Pigs keeping cool in dry Suffolk
BBC Breakfast reporter Ben Boulos has been talking to pig farmers at the Euston Estate in Suffolk to find out how they are coping with the dry weather.
East Anglia has had 67% of its normal rainfall so far this year - 185mm (7.2in) - making it the driest January-to-June period since 1996.
The River Waveney between Norfolk and Suffolk is running at about 30% of its normal flow.
Chris Fogden, from Euston Farms, says: "The pigs get quite stressed in this weather so we do everything possible to let them keep cool [such as having] insulated huts, plenty of water, and not handle them unnecessarily.
"We've got a very good private borehole system so we're not reliant on mains water and we supplement what comes out of the pipe with a big tanker."
Smoking banned on Cromer Pier
Smoking has been banned on Cromer Pier while the hot weather continues.
In a Facebook post, North Norfolk District Council says: "During this extended period of dry, hot weather there will be no smoking permitted on Cromer Pier."The timber boards, when warm and dry, are a fire risk from the embers and discarded cigarette butts."
We must be robust to drought, says water company
A company serving 4.3 million people says it is taking "the long term view" to ensure future supplies.
Read moreSend us your pictures
The BBC's Martin Barber took this picture in West Runton on the Norfolk coast at the weekend.
If you're out and about and would like your heatwave photos included on this page, you can email them to us here.
Cambridge: Will the UK's highest-ever temperature be beaten?
It's the place where the UK's highest temperature on record was logged.
On 25 July 2019, the UK saw a new record maximum temperature of 38.7C (101.7F) at Cambridge University Botanic Garden.
It beat the previous UK record of 38.5C (101.3F), set in Kent in 2003.
A Met Office official checked the equipment in central Cambridge and verified the new record.
At the time, staff at the garden tweeted to say they felt like they had "melted".
As temperatures are predicted to soar this week, all eyes will be on Cambridge, to see if this will be the place to break its own searing record.
Red weather warning: How are schools affected?
Some schools are preparing to close completely or early today and relax uniform rules to cope with rising temperatures.
Most schools will contact parents directly to let them know how children will be affected but some councils also have information on their websites.
Milton Keynes Hospital: Outpatient appointments and surgery cancelled
Routine outpatient appointments and surgery will not go ahead at Milton Keynes University Hospital on Monday and Tuesday following the warning over extreme heat.
Chief executive Joe Harrison says the NHS trust took the decision because many patients who would be travelling to these appointments are "frail and at increased risk" and the "unpredictable nature of very high temperatures" would affect "demand for emergency care and the care environment".
"We will do all we can to keep wards and departments as cool as possible, but we know this will be a challenge given the very high outside temperature," he said.
What does the red weather warning mean?
A red extreme heat warning has been issued by the Met Office as temperatures could hit 40C (104F).
Its highest warning covers an area including London, Manchester and York on Monday and Tuesday and a national emergency has been declared.
It is the first time a red heat warning has been issued for parts of the UK, although the extreme heat warning system was only introduced in 2021.
This is what the Met Office said people should expect:
M1 'expected to remain closed'
Police are asking people to avoid the M1 in Northamptonshire during today's heat as it has been closed following a multi-vehicle collision.
Both carriageways were shut after the crash between junction 16 for Northampton and junction 18 for Daventry, which happened at about 03:30.
The force says the road is "expected to remain closed throughout the morning" and diversions are in place.
In a tweet, it said: "Due to the extreme temperatures expected in the county today, we would ask all drivers to avoid the area and to seek an alternative route."
Prof highlights climate change in hot-spot of Cambridge
BBC Breakfast on BBC One has been live in Cambridge this morning.
The city's Botanic Garden had the UK's highest-ever recorded temperature in 2019, when the mercury hit 38.7C (101.7F).
Prof Emily Shuckburgh, director of Cambridge Zero at the University of Cambridge, says the country is not geared up for a Mediterranean climate and we need to tackle climate change.
"The biggest thing we could be doing is to reduce our emissions in the first place, because that is how we stop climate change," she says.
Firefighters tackle 400-acre field blaze
A fire at Great Chesterford train station affecting 400 acres of Littlebury field is one of a number at the weekend.
Read moreWildfire warning as field partially destroyed
About 40 firefighters tackle a blaze in a field in Culpho.
Read moreUK heat: Monday and Tuesday compared to the last 60 years
The year 1976 ranks as having the second-warmest summer in England on record, however the highest temperature recorded then was 35.9C (96.62F).
Dan Holley from Weatherquest said these maps highlight the difference in extent and intensity of the heat between 1976 and the highest temperature recorded in the past 60 years - compared to forecasts for today and tomorrow.
He said: "We are expecting some incredibly high temperatures here in the East, potentially as high as 40C (104F) on Monday and Tuesday.
"On top of that the nights will be very warm as well, some places may not get below 20C (68F)."
The hot summer of 1976 saw water-rationing and it's the go-to year when talking about heatwaves for most people. Here are some BBC features about it:
Whipsnade Zoo: 'Some areas may be closed'
Whipsnade Zoo said it would be open on Monday and Tuesday, but said it may need to close "some indoor areas", some animals may not be on show and some talks may be cancelled.
It advises people who are visiting to "please drink plenty of water and wear sunscreen".
In a Twitter post, it says: "The welfare of our animals, colleagues and visitors is paramount.
"Our animals have plenty of choice about where they go and what they do and many will have 'cool zones' to retreat to which aren't visible to visitors.
"Some exhibits may be closed to manage the temperature inside."
Good morning
A red extreme heat warning has been issued by the Met Office for Monday and Tuesday - as temperatures are forecast to hit up to 40C (104F).
A national emergency has been declared and people are preparing to try to keep as cool as they can.
As temperatures rise, we'll be here to show you how the East of England is being affected.
People have been up very early this morning to take their dogs out while the ground is still relatively cool - such as in St Albans in Hertfordshire (below).