Britain will go into heatwave meltdown this week with schools closing early, roads melting and trains cancelled as the country braces for its hottest day in history.The Met Office has today issued a rare red 'danger to life' extreme heat warning for 'significant disruption to daily life' as the UK faces major delays to road, rail and air travel.As temperatures climb over the next few days to a near record-breaking 40C and indoor spaces become unbearable, schools could shut amid fears over the impact of overheating classrooms on children's health.Some headteachers have already confirmed the school day will finish at lunchtime, while others are letting pupils come in wearing PE kits and cancelling sports days.Meanwhile the RAC urged drivers without air-conditioned vehicles to postpone non-essential car journeys, while the AA said motorists could see gritters out in case surfaces soften or become sticky under the heat and even rut under heavy traffic.And on the trains, operators are planning to alter some services and introduce temporary speed restrictions as the heat causes overhead lines to expand and sag and rails to buckle. Rail passengers were furious at the schedule changes, with one saying on X: 'More notice would have been appreciated, given it's been known for over a week that temperatures would be over 30C.'Another wrote: 'It is inevitable that the rail network will be subject to delays and cancellations, the railway simply isn't built for this sort of heat.' And a third tweeted: 'Britain and the weather. Heat expands and contorts rail tracks. Total disruption. Two inches of snow falls. Total disruption.' A woman sunbathes at Richmond Green in south-west London during the heatwave todayPedestrians walk over Westminster Bridge today as the heatwave continues to hit London Sunbathers bask in the hot sunshine on the beach at Lyme Regis in Dorset this afternoon A woman uses a fan to stay cool in London today as temperatures soar during the heatwaveGreat Western Railway will be running a revised service between London Paddington and Didcot Parkway after Network Rail said it would not move some points to avert the risk of them failing – meaning Slough, Maidenhead and Twyford will not be served.LNER, which runs between London and Edinburgh, is 'introducing temporary travel easements to support customers in completing their journeys more comfortably'.The Met Office said 'population-wide adverse health effects are likely' while 'failure of heat-sensitive systems and equipment is likely with the probable loss of power and other essential services, such as water, electricity, gas or mobile phone services'.The extreme heat will be accompanied by high humidity, and very warm and humid nights which will make it hard for people to recover overnight. The Met Office also urged people to adapt their routines where possible to cope with the weather.The red warning is only the second ever, and will cover parts of central and southern England as well as Wales from 9am on Wednesday to 9pm on Thursday. A separate wider amber heat warning began today and will run until at least the end of Thursday.One climate scientist said the 'heat dome driven furnace' will bring 'truly exceptional' temperatures, while a Met Office meteorologist called it a 'very serious situation'.The UK Health Security Agency (UKHSA) meanwhile has upgraded its heat health alerts from amber to red for Wednesday and Thursday for much of England. Other amber heat health alerts will continue to be active from now until the end of Friday.Red alerts warn of 'increased risk to life across the whole population, with significant impacts on older people'; 'significantly increased demand on all health and social care services' and the 'heat affecting the ability of the workforce to deliver services'.There are also concerns about the impact on animals, and the British Horseracing Authority has already abandoned four meetings. Temperatures in southern England could hit 34C today then 37C tomorrow before peaking at 40C on both Wednesday and Thursday, followed by 33C on Friday. HOTTEST DAYS IN UK HISTORY 1 40.3C July 19, 2022 Coningsby, Lincolnshire 2 38.7C July 25, 2019 Cambridge 3 38.5C August 10, 2003 Faversham, Kent 4 38.2C July 18, 2022 Pitsford, Northamptonshire 5 37.8C July 31, 2020 London Heathrow 6 37.1C August 3, 1990 Cheltenham, Gloucestershire 7 36.7C July 1, 2015 London Heathrow 8 36.7C August 9, 1911 Raunds, Northamptonshire 9 36.6C August 2, 1990 Worcester 10 36.5C July 19, 2006 Wisley, Surrey The UK's all-time high of 40.3C was set in July 2022, followed by 38.7C in 2019 in second; 38.5C in 2003 in third; 38.2C in 2022 in fourth; and 37.8C in 2020 in fifth.The highest ever June temperature is 35.6C both in London in 1957 and Southampton in 1976 – but this could be broken three times this week, and as early as tomorrow.Humid conditions are also forecast for the country alongside tropical nights, when the overnight temperature does not fall below 20C, making it feel muggier than last month's heatwave – which itself brought the hottest ever day in May at 35.1C.A record-breaking June day would be the first time since 1911 that two consecutive months have observed record temperatures. Brits are also struggling to get hold on portable air conditioning units amid a rush in demand. One X user tweeted: 'If you're in the UK all of the portable AC units are sold out from all of the stores. I've checked about buying an additional unit since last Thursday.'Despite the increasing number of heatwaves in recent years, the Department for Education has no official upper limit on the indoor temperature for schools.But the UK Government has been considering recommendations from the Climate Change Committee that indoor classrooms should be no higher than 25C.The National Education Union says 26C should be a maximum and its heatwave plan for teachers states schools could close under a red warning for extreme heat.Some schools are allowing students to come in wearing PE kits while others have confirmed they will operate with shorter days to protect the health of staff and pupils.New Close School in Warminster, Wiltshire, said its site would close early at 12.30pm each day between today and Thursday 'due to the forecast of extreme heat'.A statement said: 'This decision has not been made lightly and is in advance of the expected peak temperatures, with the safety and wellbeing of both children and staff as our highest priority. The Met Office has issued a red warning for extreme heat on Wednesday and Thursday The UK Health and Security Agency red heat health alerts for Wednesday and Thursday