Britain faces a red 'danger to life' warning for extreme heat this week during an extraordinary 40C heatwave which could bring the country's hottest day in history.The Met Office warning for 'significant disruption to daily life' means the UK could endure major delays to road, train and air travel as surfaces melt and rails buckle.Forecasters 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'.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.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. A woman sunbathes at Richmond Green in south-west London during the heatwave today The Met Office forecast for London shows temperatures of 39C for Wednesday and Thursday 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