The UK is set to see its hottest day on record, with temperatures on Tuesday expected to reach up to 42C (107.6F).
A high of 38.1C was reached in Suffolk on Monday, just short of the UK record of 38.7C set in 2019. Wales recorded its hottest day on record with 37.1C.
The Met Office has issued a red extreme heat warning covering much of central, northern, and south-east England.
At least four people are believed to have drowned after attempting to escape the heat in rivers and lakes.
Network Rail has issued a "do not travel" warning for Tuesday affecting services travelling through the "red zone" of the Met Office's weather warning.
The extreme warning, indicating a threat to life, is in place in an area stretching between London, Manchester and York.
Peterborough, Caernarfon and Swindon are among the areas projected to reach 32C by 10:00 BST, according to the Met Office forecast.
By 16:00, Lincoln, Cambridge and Huntingdon could see 40C - areas in the A1/M1 corridor may surpass this.
No Thameslink or Great Northern services are scheduled to run north from London all day and there will be no services from London King's Cross or on the East Coast mainline.
There will also be only limited services from London Euston, London Marylebone and on the East Midlands Railway.
The East Midlands Railway is only running very limited services between Derby, Nottingham, Luton, Bedford and London, which will stop altogether during the hottest part of the day, from lunch time to 19:00.
Rail services on Monday evening were heavily impacted by the extreme heat, Network Rail said, with buckled rails reported and overhead wire systems failing. A new record rail temperature of 62C was recorded in Suffolk.
Jake Kelly, the group director for system operation at Network Rail, said it took "the difficult and regrettable" decision to close the East Coast Mainline and the Midland Mainline due to record temperatures.
Mr Kelly told BBC Radio 4's Today programme: "We don't take decisions like this lightly. Our engineers work very hard assessing the capability of the infrastructure facing that record heat, and we decided that we had no choice but to close it."
There have also been warnings of pressure on hospitals and ambulance services as temperatures are set to peak on Tuesday afternoon.
Following the government's latest emergency Cobra meeting, Health Secretary Steve Barclay said more call handlers had been put in place and additional funding made available for ambulance and 111 services.
Monday saw a number of schools close despite government advice against doing so, although one teaching union said the majority of schools had remained open.
Water companies in southern and eastern England have warned increased demand is leading to low pressure - and even interrupted supply - for some households.
Heatwaves are becoming more likely and more extreme because of human-induced climate change.
The world has already warmed by about 1.2C since the industrial era began, and temperatures will keep rising unless governments around the world make steep cuts to emissions.
The peak temperature reached on Monday made it the third-hottest day on record and the hottest of the year so far.
A temperature of 37.1C was measured in Hawarden, Flintshire, making it the hottest day on record for Wales, according to provisional figures from the Met Office.
Scotland and Northern Ireland also saw their warmest days of the year, with temperatures of 31.3C and 31.1C recorded in Aboyne, Aberdeenshire and Derrylin, Co Fermanagh respectively.
Temperatures remained in the low and mid-20s overnight, raising the possibility the UK would also see its warmest night on record.
Responding to suggestions the UK has seen worse heat - such as during the prolonged heatwave in 1976 - BBC Weather's Simon King said the "dangerously high" expected temperatures were up to 10C above the extended heatwave and severe drought experienced then.
The public are being warned to take care if they cool off in water following a number of deaths.
On Monday, a 16-year-old boy died after getting into difficulty in Maidenhead, Berkshire, while the Metropolitan Police said a 14-year-old boy went missing and is believed to have drowned after entering the Thames at Tagg's Island in Hampton, south-west London.
Those incidents followed the deaths of a 16-year-old boy swimming in Salford Quays on Saturday evening and a 13-year-old boy who went missing in the River Tyne in Northumberland on Sunday.
Monday marked the first time the Met Office has issued a red warning since the system was introduced last year.
It indicates that widespread adverse health effects, not limited to those already vulnerable to extreme heat, are expected, and that "substantial changes in working practices and daily routines will be required".
Much of Europe and North Africa are also experiencing extreme heat with wildfires breaking out in France, Spain, Portugal, Greece and Morocco.
Source: BBC
Image source: MSBM
