BRITS have been warned not to travel and to work from home while the nation's extreme heat alert is in place.
The Met Office issued a red "danger to life" weather warning today ahead of a three-day scorcher this weekend.
Its forecasters expect temperatures to climb to the mid-30s across much of England over the coming days.
Brits were cautioned over an "exceptional hot spell on Monday and Tuesday leading to widespread impacts on people and infrastructure".
And Network Rail has since warned people to travel only if absolutely necessary, with speed restrictions and disruption likely.
Jake Kelly, Network Rail's System Operator group director, said: "Rail passengers in England and Wales should only travel if necessary on Monday and Tuesday as there will be delays and cancellations to train services due to the unprecedented heat we're expecting.
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"The wellbeing of our passengers is our first priority so we're asking all passengers who decide to travel to take time to prepare before leaving the house."
He added: "Journeys will take significantly longer and delays are likely as speed restrictions are introduced to keep passengers and railway staff safe, so make sure to allow considerably more time to complete your journey and be prepared for very hot conditions."
The Met Office issued its red warning this morning, declaring the heat blast could spark "population-wide adverse health effects" adding it could lead to "serious illness or danger to life".
It also suggested people would need to work from home, insisting "substantial changes in working practices and daily routines will be required".
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A spokesperson for the Prime Minister said working conditions would be a matter for "individual employers" to decide.
And they added that Network Rail and Transport for London (TFL) would "monitor" the situation.
Meanwhile, Penny Endersby, Met Office chief executive, said: "The extreme heat that we're forecasting right now is absolutely unprecedented.
"Please treat the warnings we are putting out as seriously as you would a red or amber warning from us for wind or snow and follow the advice.
"Stay out of the sun, keep your home cool, think about adjusting your plans for the warning period."
Britain has basked in sweltering temps this week, leading the UK's Health Security Agency Heat Health Alert to reach Level 3 - which means that temperatures are likely to have a high impact on public life.
Tuesday is likely to be the hottest day during this heatwave, with the potential to top the UK record temperature of 38.7C (101.7F) set in Cambridge in 2019, forecasters said.
Grahame Madge, Met Office spokesman, said today: "We've just issued a red warning for extreme heat for Monday and Tuesday which is the first such warning ever issued.
"The warning covers an area from London up to Manchester and then up to the Vale of York."
Avanti West Coast has warned their train services may be cancelled at the last minute, while TransPennine Express (TPE) urged passengers taking essential journeys to pack water and sun cream.
Residents of the South and South West can expect warm sunshine while outbreaks of showery rain will move south-east across northern and central areas of the UK.
London is forecast to experience 26C on Friday, while 23C is predicted for Cardiff, 20C in Belfast and 19C in Edinburgh, according to the Met Office.
Saturday is also set to start with overcast skies and scattered showers in northern areas, before turning dry across the nation as the extreme heat moves in.
During the peak of the heatwave on Tuesday, 36C is predicted in the capital, 28C in Cardiff, 26C in Belfast and 25C in Edinburgh.
Cabinet Office minister Kit Malthouse has said the Government is preparing for a "surge" in demand on a number of services.
After chairing a meeting of the Cobra civil contingencies committee in Whitehall, Mr Malthouse urged the public to look out for people who were particularly vulnerable in the heat.
Avanti West Coast said weekend services may be cancelled at short notice while journeys on Monday and Tuesday will be subject to amended timetables.
It was understood that closing schools was not being discussed as part of the contingency measures during the heatwave.
Network Rail said response teams will be deployed to mitigate the impact of the extreme heat where possible, but passengers are likely to experience disruption if current forecasts are realised.
England's chief medical officer, Professor Sir Chris Whitty, asked people on Twitter to look out for each other.
He said: "The Met Office has issued an extreme heat warning for next week.
"Please remember the signs of heat exhaustion and heatstroke especially for older and medically vulnerable people.
"Early intervention to cool people down and rehydrate them can be lifesaving."
At the start of next week, people should, if they are travelling, take water with them, Commons Deputy Speaker Nigel Evans has said.
The Department for Digital, Culture, Media and Sport (DCMS) will be giving advice about Euro 2022 matches being held during the heatwave emergency.
Discussing contingency measures, a No 10 spokesman said: "Another example is DCMS is engaged with sporting sector bodies and the organisations of major events taking place next week, such as the Women's Euros."
Asked if it could lead to matches being cancelled, the spokesman said: "I believe it will be more advisory, the organisation of the event is for the organisers, but DCMS will engage with sporting authorities."
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A No 10 spokesman said: "Cobra met yesterday and officials from across Government will continue to meet regularly both today and throughout the weekend."
Asked if the PM is involved, the spokesman said: "As always the Prime Minister is kept up to date with all the latest information."
What to expect in a red warning for extreme heat?
- Population-wide adverse health effects experienced, 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; seek advice from 111 if you need non-emergency health advice.
- Substantial changes in working practices and daily routines will be required
- 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, with significant welfare issues for those who experience even moderate delays