The Met Office has warned that temperatures could rise even further to 43C (109F) tomorrow - with trains already cancelled, hospital appointments cancelled and schools closed amid warnings that healthy people could die.

Its chief executive Penny Endersby confirmed 'we may well see the hottest day in the UK in history' today - beating the record of 38.7C (101.7F) set in Cambridge in July 2019 - but tomorrow is now expected to be even hotter.

With the UK set to be hotter than the Sahara Desert today, transport links in London were already grinding to a halt due to train cancellations - while roads could melt and bosses have urged employees to work from home.

Health chiefs told patients to stay away unless it is an emergency amid fears hospitals will be overwhelmed, while emergency services urged swimmers to stay away from lakes and rivers in case they face difficulties.

Some schools in Nottinghamshire, Hampshire and Oxfordshire have shut while others will close early - and water providers have also warned of shortages after the hottest day of 2022 so far yesterday with 33C (91F) highs.

Other schools were cancelling detentions and sports days because of the heatwave.

Northwood Community Primary School in Kirkby, Merseyside, said that sports day had been axed today; while King Charles I School, a secondary school in Worcestershire, has cancelled all on-site detentions both today and tomorrow.

Professor Endersby told BBC Radio 4's Today programme this morning: 'We think today we may well see the hottest day in the UK in history, with the hottest temperatures in the South East, but actually the highest temperatures we expect tomorrow, and those temperatures will be further north as that warm air pushes north. 

'It's tomorrow that we're really seeing the higher chance of 40C and temperatures above that.

Even possibly above that... 41C isn't off the cards. We've even got some 43Cs in the model but we're hoping it won't be as high as that.'

One GP surgery in Hertfordshire had to close a site today because it has no air conditioning; others in London have texted patients to warn them of reduced services with limited clinic rooms in operation; and Milton Keynes University Hospital said it was 'standing down routine outpatient appointments and surgery' today and tomorrow. 

In Cardiff, a children's hospital's cancer ward at Noah's Ark Children's Hospital was left without air conditioning after the unit failed in hot weather.

Engineers were working to fix the fault affecting the chemotherapy area - and health chiefs said that if the problem cannot be solved, patients will be moved to a different ward to keep cool.  

As health officials declared a 'national emergency', rail chaos was already affecting parts of London this morning - with the Overground suspended between Willesden Junction and Richmond, and Romford and Upminster.

On the Underground, the District, Central, Bakerloo and Jubilee lines all had severe delays while the Hammersmith & City Line was completely axed due to 'heat related restrictions' and there was no Metropolitan line between Baker Street and Aldgate.

Transport for London told all passengers in the capital to avoid non-essential travel.

The scorching heat means the UK will be warmer than Nassau in the Bahamas (32C), Kingston in Jamaica (33C), Malaga in Spain (28C), Athens in Greece (35C), Albufeira in Portugal (28C) and Dakhla in the Western Sahara (24C).

Temperatures had already hit 31C (88F) at Southend-on-Sea in Essex by 10.30am today.

As Britons camped overnight at Bournemouth beach for the best spot today amid what forecasters called an 'exceptional hot spell':

• The Met Office urged people to do 'as little as possible' to avoid dire health risks as the 'red warning' began;
• Rail passengers were urged to travel only if 'necessary' and gritters were sent out to stop roads melting;
• Wildfires swept through parched grassland after days of dry and roasting conditions in 'tinderbox' Britain;
• There is now a 90 per cent chance of the all-time UK temperature record being broken today or tomorrow;
• Water providers including Affinity, Anglian and South East reported supply issues due to the hot weather.

Professor Endersby said today that such extreme temperatures are not expected beyond tomorrow, but that the Met Office will then be monitoring the possibility of drought in the coming months. 

'Well, we certainly don't see these very hot temperatures persisting past Tuesday, so we're expecting a big drop in temperature, mercifully, overnight into Wednesday - down 10 or 12 degrees on what it has been the days before.

'We are still seeing hotter than average in our three-month outlook and also very dry, and our attention is turning, once we're past these two days, to drought and when we might see any rain, and we're not seeing any significant rain coming up.'

She also said that the public should take Met Office heat warnings as seriously as those about other significant weather events such as snow or wind, as the extreme heat could cause thousands of excess deaths.