As 'Furnace Britain' braces for sweltering conditions this week, a scientist has warned that temperatures could hit 43°C this summer.Parts of England and Wales are expected to reach record highs of 40°C on Wednesday and Thursday amid a four–day extreme heat warning issued by the Met Office.It said large areas will also experience 'exceptionally warm' conditions throughout the evenings, with tropical nights – where the temperature does not drop below 20°C – likely for many.This has prompted a series of health alerts, as extreme heat can trigger serious illness or danger to life.While such temperatures would be unprecedented for this time of year, a scientist has warned they could be set to get even higher.Bill McGuire, Professor Emeritus of Geophysical & Climate Hazards at University College London, said: 'The UK temperature broke the 40°C barrier in July 2022, so it should come as no surprise that – after another four years of global heating – here we are again.'In the current climate, UK summer temperatures of 43°C+ are possible, as are heatwaves lasting for multiple days with temperatures at or close to 40°C.'This has massive implications for health, energy infrastructure, and transport, which are simply not built for these conditions.' The Met Office forecast shows a sweltering 40°C is expected in London on Wednesday. The UK's all–time high of 40.3°C was set in July 2022 Parts of England and Wales are expected to reach record highs on Wednesday and Thursday amid a four–day extreme heat warning issued by the Met OfficeThe highest ever June temperature currently stands at 35.6°C, both in London in 1957 and Southampton in 1976.The Met Office said this could be broken as early as tomorrow, as Britain faces its second hottest day in history.Temperatures in southern England could hit 34°C today then 37°C tomorrow before peaking at 40°C on Wednesday, 39°C on Thursday, followed by 33°C on Friday.The UK's all–time high of 40.3°C was set in July 2022, followed by 38.7°C in 2019.This means a high of 40°C and 39°C on Wednesday and Thursday – mostly likely in London or the Home Counties – would make them Britain's second and third hottest days ever.An amber warning for extreme heat, which has been issued for the first time in four years, began early this morning and will now run until at least Thursday. The UK Health Security Agency (UKHSA) has issued amber heat health alerts until the end of Friday which now cover almost all of England.Experts also say there could be major disruption to road, train and air travel, with the possibility of surfaces melting and rails buckling.'As 40°C+ temperatures become ever more common, expect many thousands sleeping in the streets as poorly insulated homes become uninhabitable heat traps, widespread power cuts as power cables sag and break, transport chaos as rails, overhead wires and signalling fail, and A&E departments overwhelmed by the old, very young, and vulnerable suffering from overheating,' Professor McGuire said. Spectators sit in the sun during the match between Australia's Alex de Minaur and Brandon Nakashima of the USA at the HSBC Championships at Queens Club in London People enjoying the hot weather on Bournemouth beach in Dorset over the May bank holiday last month 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 Humid conditions are also forecast for the country, making it feel muggier than last month's heatwave – which itself brought the hottest ever day in May at 35.1°C.A static area of high pressure over mainland Europe is trapping warm air underneath – known as a 'heat dome' – and resulting in very hot conditions for southern England.The extreme amber heat warning flags potential health impacts for people vulnerable to extreme heat and heat–related issues, as well as a possible increase in water safety incidents.Today and tomorrow's warning covers most of southern England, central and eastern Wales, and much of the Midlands, while Wednesday and Thursday's alert also includes parts of Greater Manchester, Yorkshire and Cheshire.People in the alert area are advised to drink plenty of fluids and keep out of the sun, and avoid exercising between 11am and 3pm, the weather service said.Richard Allan, professor of Climate Science at the University of Reading, said: 'A heatwave in June is par for the course but temperatures nudging toward 40°C were unprecedented for the UK up until 2022.'Whilst May brought record dry heat, this week will see a more muggy heatwave that makes it difficult to stay cool and critical for those with underlying health conditions.'It is blindingly obvious that heatwaves will increase in severity as rising greenhouse gases stifle the planet's ability to lose heat to space.'