Temperatures had risen to 37.5C in Kew Gardens, west London, by mid-afternoon on Monday, making it the hottest day of the year, and the mercury topped 37C in a number of other places.
The Met Office was forecasting temperatures could climb to 38C or even a record-breaking 39C on Monday.
Wales has provisionally recorded its hottest day on record, with the temperature reaching 35.3C in Gogerddan, near Aberystwyth, beating the previous record for the country which has been in place since 1990.
Tuesday is predicted to be even hotter, with temperatures possibly reaching a “crazy” 41C in Yorkshire and Lincolnshire.
The existing UK record, of 38.7C, recorded in Cambridge in 2019, looks certain to fall amid the sweltering heat.
Met Office meteorologist Luke Miall said: “For a good part of eastern Wales, and England and southern Scotland, we’re looking at the 30s if not the high 30s.” He added that London could see temperatures hit 40C.
He said: “I’ve been a qualified meteorologist for 10 years, and telling people about 41C in the UK doesn’t seem real.
“It’s crazy how we are talking about these sorts of values, I’ve never seen the models coming up with these values.
“It’s been quite an eye-opener to climate change with all these temperatures in the UK.”
Climate change, which has pushed up global temperatures by 1.2C on pre-industrial levels, is making heatwaves longer, more intense and more likely.
Experts have warned of the need to adapt homes, cities and infrastructure in the UK for a future of more intense summer heat.
Hot air from Europe is contributing to the extreme heat in Britain, with a searing heatwave baking much of the continent, fuelling fierce wildfires in France and Spain.
The UK Health Security Agency has issued a level four heat-health alert – described as an “emergency” – and the UK is under its first red extreme heat warning for a large part of England, issued by the Met Office.
Reduced railway services and slower running times of trains have been put in place amid fears of rails buckling in the heat, with customers being advised to only travel if necessary on Monday and Tuesday.
Network Rail said the number of passengers using major stations across Britain on Monday was around 20 per cent down on a week ago, and road traffic was also down.
In London, congestion levels fell from 53 per cent on July 11 to 42 per cent on Monday.
There were also reduced services on the Tube and buses, as people heeded the advice to avoid travel if possible.
Some schools in several counties, including Nottinghamshire, Buckinghamshire and Hampshire, were closed, while others cancelled sports days and detentions, and relaxed uniform codes.
Additional contingency support is in place for ambulance services, such as more call handlers and extra working hours, with calls to the service expected to increase by thousands.
Britons are being urged to stay inside during the hottest period of the day, between 11am and 4pm, and wear sun cream, a hat, stay in the shade and keep hydrated with water, and there are warnings about swimming in lakes, rivers and reservoirs with four people thought to have died getting into trouble in open water.
Local News Matters
Media diversity is under threat in Australia – nowhere more so than in South Australia. The state needs more than one voice to guide it forward and you can help with a donation of any size to InDaily. Your contribution goes directly to helping our journalists uncover the facts. Please click below to help InDaily continue to uncover the facts.