Train services have been cut, schools closed and ambulance crews are braced for a rise in 999 calls as temperatures soar into the 30Cs in the UK.
Temperatures had risen to 37.5C in Kew Gardens, west London, by 3pm, making it the hottest day of the year, and the mercury topped 37C in a number of other places.
The UK Met Office was forecasting temperatures could climb to 38C or even a record-breaking 39C today.
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.
"For a good part of eastern Wales, and England and southern Scotland, we're looking at the 30s if not the high 30s," said Met Office meteorologist Luke Miall, adding that London could see temperatures hit 40C.
"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.
In Ireland, Met Éireann said a provisional temperature of 33C was reported from the weather station at Phoenix Park in Dublin at 1pm today.
If confirmed, it will be the highest temperature ever recorded in Ireland in July and breaks the high temperature records for the 21st and 20th century.
The UK health security agency has issued a level 4 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.
The government is holding another emergency cobra meeting to discuss the response to the heat.
London's Luton Airport has reopened to flights after a runway defect caused by the hot weather, but said arrivals remain suspended until further notice.
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 today and tomorrow .
Network Rail said the number of passengers using major stations across Britain on Monday was around 20% down on a week ago.
There were also reduced services on the Tube, which saw around 18% fewer journeys compared to the same time the week before, as people heeded the advice to avoid travel if possible.
Some 1.07 million bus journeys were made up to 10am, a 10% decrease week on week, Transport for London said.
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, but health bosses warned the NHS will be hard pushed in the face of the extreme heat.
Interim deputy chief executive of NHS Providers, Miriam Deakin, said the entire health service is under pressure as temperatures rise, with some operating theatres getting too hot, leading to surgery being cancelled.
She added: "People with underlying conditions, older people and children tend to be most at risk.
"And while there have been additional steps put in place to increase ambulance capacity, ambulance trusts will be under significant pressure as the number of 999 calls can be expected to rise."
Brian Jordan, director of 999 operations at London Ambulance Service, urged people to only dial 999 in an emergency as he told the BBC a busy day would see 5,500 emergency calls to the service but he was anticipating up to 8,000 today.
People are being advised to 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.
Officers searching for a missing 13-year-old who got into trouble in a river near, Northumberland, have found a body, Northumbria Police said, while a 16-year-old boy died after getting into difficulty in Bray Lake, near Maidenhead, Berkshire, Thames Valley Police said.
The body of a 50-year-old man was recovered from Ardsley Reservoir, between Leeds and Wakefield, on Sunday.
There are also warnings of wildfires, with people warned not to use barbecues or leave litter that could spark fires in the countryside, while zoos and wildlife parks are closed to protect animals, staff and visitors.
Water companies have been experiencing "unprecedented peak demand", with people encouraged to "carefully consider" their water usage and urged not to waste it.
