The UK has experienced one of its hottest ever days, putting pressure on transport and health services, with the Met Office warning of worse to come.

The mercury hit 38.1C in Santon Downham, Suffolk, making it the hottest day of the year and the third hottest day on record, after 38.7C in Cambridge in 2019 and 38.5C in Faversham, Kent, in August 2003.

The current UK temperature record looks almost certain to fall on Tuesday, when temperatures could reach a “crazy” 41C in Yorkshire and Lincolnshire, and hit 40C in London.

It comes as a 14-year-old boy is missing and believed to have drowned on Monday afternoon after getting into difficulty in the Thames in Richmond, west London.

The teenager was seen entering the water at Tagg’s Island in Hampton and, after a search took place, he was presumed dead, with officers calling the incident a “tragedy”.

Rail services were reduced and trains were forced to travel more slowly amid the risk of buckling rails, and people appeared to heed warnings to work from home as traffic, train and Tube passengers were down on the previous Monday.

There were warnings of pressures on hospitals from the extreme temperatures, and concerns ambulance services would face rising numbers of calls as the heat peaks on Tuesday afternoon.

Luton Airport runway was closed while engineers worked to repair a “surface defect”, and flying activity was also halted at RAF Brize Norton.

Wales has already provisionally recorded its hottest day on record, with the temperature reaching 37.1C Hawarden in Flintshire, beating the previous record for the country which has been in place since 1990 by almost 2C.

On Tuesday, Scotland could see its hottest day on record.

The Met Office is also warning that temperatures overnight into Tuesday are holding up in the low 20s and possible mid-20s in isolated places, and it looks likely to be the warmest night on record in the UK.

Met Office meteorologist Luke Miall 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 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.

Ministers have defended Boris Johnson against opposition accusations that he has already “clocked off” and is failing to take the lead on the Government’s response to the situation.

Following the Government’s latest emergency Cobra meeting, Health Secretary Steve Barclay said the emergency services were already starting to see an increased volume of calls, but Tuesday afternoon was the “point of maximum concern” as the highest expected point of the heatwave.

More call handlers have been put in place, along with additional funding for ambulance services, 111 and auxiliary ambulances, he said.

Several ambulance services told the PA news agency they were coping as normal and the NHS’ medical director said the vast majority of NHS services were running.

But earlier interim deputy chief executive of NHS Providers, Miriam Deakin, said the entire health service was 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.”

Meanwhile, visits to the heat exhaustion section of the NHS website have increased by 525% in the past week.

Dozens of schools have closed, despite Government advice to stay open, while others cancelled sports days, school trips and detentions, and relaxed uniform codes.

Britons have been 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.

The family of Robert Hattersley, 13, of Crawcrook, who died following an incident in the River Tyne near Ovingham in Northumberland, paid tribute to the teenager, saying he “brought a smile to so many people’s faces and he will be missed by absolutely everyone who knew and loved him”.

A 16-year-old boy died after getting into difficulty in Bray Lake, near Maidenhead, Berkshire, Thames Valley Police said, while 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 were 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.