Around 50 schools across East Anglia have announced they will close or reduce hours, while travel services have been disrupted as extreme heat builds in the region.
The move comes after the Met Office issued a red "risk to life" warning for Monday and Tuesday in many parts of the East of England and the rest of the British Isles.
Temperatures are set to break UK heat records.
In Cavendish in Suffolk and Writtle in Essex the temperature hit 35.5C (96F) before noon, making it the UK's hottest day of the year already.
Identical readings were taken at three London locations and one in Surrey at the same time.
By 5.15pm, the country's highest temperature had been recorded as 38.1C at Santon Downham in Suffolk at 2.51pm.
The country's highest ever temperature of 38.7°C (101.7°F), set in Cambridge just three years ago, will most likely be broken later with forecasters predicting thermometers could hit 40°C (104°F) or even 41°C (106°F).
Police in Hertfordshire say they arrested a man who left his dog in a car at South Mimms Service Station with all the windows and doors shut. The RSCPA are being contacted and the dog is having emergency treatment.
ITV News Anglia meteorologist and weather presenter Chris Page warned: "Not only is the UK all time maximum temperature record likely to be broken, so is the highest overnight low.
"Very hot days and warm sticky uncomfortable nights lie ahead. Please try and stay cool and keep yourself well hydrated."
On Monday a number of schools across the region had decided to close or reduce school hour. By county, the numbers were:
1 - Essex
4 - Suffolk
12 - Norfolk
14 - Bedfordshire
17 - Cambridgeshire
Some schools are closing because of a lack of water supply in the extreme weather conditions.
North Wootton Academy and South Wootton Junior and Infant School in King's Lynn, Norfolk, are closed because of low water supply in the heat.
The health secretary has said extra measures are being put in place for ambulance services as an extreme heat warning comes into force.
Steve Barclay, who is also an MP in Cambridgeshire, said additional contingency support, such as more call handlers and extra working hours, are being put in place for Monday and Tuesday.
Milton Keynes Hospital has cancelled routine operations and outpatients appointments on Monday and Tuesday.
The hospital said: “Temperatures of this level are likely to impact on both health and infrastructure and we have taken the view that it is better to plan for that impact by reducing routine activity now.
“Urgent appointments and procedures will continue as planned.”
The weather is also starting to cause problems on the railway.
Services to and from Peterborough, Stevenage will be severely disrupted on Tuesday as the East Coast Main Line will be closed.
Meanwhile train operator Great Northern reported that high temperatures had caused the railway to buckle at Watlington near King's Lynn. No services are running in and out of King's Lynn.
It comes as Network Rail urged train passengers to travel only if absolutely necessary.
It said: "There will be delays, cancellations and last-minute changes to train services due to the unprecedented record heat on those days. Hot weather can affect the rails, overhead power lines and the ground the track sits on.
"On very hot days, we work hard to get you to where you need to go, safely and on time by minimising the impact of hot weather on the railway."
The rail company Greater Anglia said journeys are expected to take longer, as trains will be travelling at slower speeds than usual and service alterations will take place on many routes.
Passengers are warned to expect widespread disruption and short-notice cancellations, especially from the middle of the day onwards when the hottest temperatures occur.
Government ministers held a virtual emergency Cobra meeting on Saturday after meteorologists warned the record high temperatures could put lives at risk.
Cabinet Office Minister Kit Malthouse, who chaired the meeting, said transport services would face “significant disruption” on Monday and Tuesday and urged people not to travel.
He added schools were being issued with guidance to enable them to remain open.
The Met Office red weather warnings states: "Population-wide adverse health effects experienced, not limited to those most vulnerable to extreme heat, leading to serious illness or danger to life."
Dr Nikos Christidis, climate attribution scientist at the Met Office, has said the 40C prediction is a result of climate change.
“We hoped we wouldn’t get to this situation, but for the first time ever we are forecasting greater than 40C in the UK,” he said.
“In a recent study we found that the likelihood of extremely hot days in the UK has been increasing and will continue to do so during the course of the century, with the most extreme temperatures expected to be observed in the south east of England.”