The UK has experienced its warmest night on record from Monday into Tuesday, according to provisional Met Office figures.
And the country has now experienced its hottest day on record, with the temperature reaching 39.1C in Charlwood, Surrey.
The mercury peaked at 38.1C in Santon Downham, Suffolk, on Monday, making it the hottest day of the year and the third hottest day on record, after 38.7C in Cambridge in 2019 and 38.5C in Faversham, Kent, in August 2003.
Brits are now braced for possible highs of 41C in isolated areas on Tuesday, making the country hotter than Jamaica, the Maldives and Barbados – with rail users warned of delays, cancellations and changes to services.
Met Office forecaster Rachel Ayers said: “Overnight it’s going to stay very warm so expect a difficult night of sleeping.
The skyline of the City of London during sunrise
Yui Mok
The sun rises from behind an apartment block in east London
Yui Mok
“We then have a pretty unprecedented day tomorrow, the temperature will be very hot throughout the day, before rising as high as 40C, maybe even 41C in isolated spots across England during the afternoon.
“This will make it the hottest day on record and the first time we have seen temperatures as high as 40C.”
Scotland and Wales could also see their hottest days on record.
The Met Office is warning that temperatures overnight into Tuesday are holding up in the low 20s and possible mid-20s in isolated places, and it looks likely to be the warmest night on record in the UK.
Most routes across England and Wales will be affected by the hot weather, according to National Rail, with customers told only to travel if “absolutely necessary”.
Network Rail said: “There will be delays, cancellations and last-minute changes to train services due to the unprecedented record heat on those days.”
Ms Ayers added: “There are likely to be delays on roads, with road closures, as well as possible delays and cancellations to trains and maybe issues with air travel.
“This could pose a significant health risk to those stuck on services or roads during the heat.”
There will be no Thameslink or Great Northern trains running in any location north of London, from London Blackfriars via St Pancras, or from London King’s Cross or London Moorgate on Tuesday.
Merseyrail said the number of trains running and journey times will be “seriously affected”, with some routes closed completely.
LNER will run no trains from south of York and south of Leeds to London King’s Cross.
Southern, South Eastern, South Western Railway and Great Western Railway are among the dozens of train companies running significantly reduced services across the country.
Transport for London (TfL) said London’s rail network would also be running a reduced service due to safety restrictions put into place to deal with the heat.
Council gritters are also on stand-by to spread light dustings of sand on melting roads.
The RAC anticipated that the number of vehicle breakdowns on Monday and Tuesday could be up to a fifth higher than normal.
Paddleboarders turn out to watch the sunrise at Cullercoats Bay, North Tyneside
Owen Humphreys
A member of the Queen's Guard receives water to drink during the hot weather
JOHN SIBLEY
The Department for Environment, Food and Rural Affairs (DEFRA) has a high pollution alert in place in the eastern parts of England and the East Midlands.
The UK Health Security Agency has issued a level 4 heat-health alert – described as an “emergency” – and the UK is under its first red extreme heat warning for a large part of England, issued by the Met Office.
Brits have been urged to stay inside during the hottest period of the day, between 11am and 4pm, and wear sun cream, a hat, stay in the shade and keep hydrated with water.
There are warnings about swimming in lakes, rivers and reservoirs.
There are also warnings of wildfires, with people asked not to use barbecues or leave litter that could spark fires in the countryside – while zoos and wildlife parks were closed to protect animals, staff and visitors.
Water companies have been experiencing “unprecedented peak demand”, with people encouraged to “carefully consider” their water usage and urged not to waste it.