The UK has recorded its highest ever May temperature for the second consecutive day, as thermometers hit 35.1C at Heathrow and Kew Gardens in London, the Met Office has said.

The latest high was recorded the day after the country’s provisional hottest meteorological spring temperature, of 34.8C in Kew Gardens in south-west London. The previous May peak of 32.8C had stood since 1922.

The Met Office issued a yellow weather warning for thunderstorms across England on Tuesday. Forecasters said isolated storms with lightning, hail and gusty winds could affect large parts of the country, from Bath and Reading to Lincoln and Sheffield.

The Met Office said many areas would stay hot and sunny but that there was potential for as much as 30mm of rainfall in the space of an hour in some parts. The warning will be in effect from 3pm to 10pm.

Earlier on Tuesday, the amber health warning was extended by 24 hours for several regions in England.

The UK Health Security Agency said amber alerts for the south-west, south-east, London, East and West Midlands and the west of England would remain in place until 5pm on Thursday, along with yellow alerts for the north-west and north-east. The alerts had previously been in place until 5pm on Wednesday, while the yellow alert issued for the south-west has been raised to amber.

The National Fire Chiefs Council issued a water safety warning after several deaths over the bank holiday weekend, including those of four teenagers.

A 13-year-old boy died on Monday after getting into difficulty in a West Yorkshire reservoir, while South Yorkshire police said the body of a boy had been found in the early hours of Tuesday after he was last seen entering the water at a park in Rotherham the day before.

In Lincolnshire, police said divers had found the body of 15-year-old Declan Sawyer, who had been missing since entering the water at Swanholme Lakes in Lincoln on Monday. That same evening, the body of a teenage girl was recovered from the water at Kingsbury water park in Warwickshire.

In Cornwall, police said a man died after trying to save two of his family members who had got into trouble in the sea at Tregirls beach in Padstow. Devon and Cornwall police said: “The family members were then brought to safety by members of the public. Emergency services and the RNLI attended the scene but, sadly, the local man in his 60s was pronounced deceased at the scene.”

On Tuesday afternoon, police in Lancashire said searches were under way after a boy got into difficulty while swimming in a river in the Ribble Valley.

The country experienced a “tropical night” on Monday, defined as when temperatures do not fall below 20C. Kenley airfield in south London recorded an overnight temperature of 21.3C.

Many places across England and Wales reached the heatwave threshold on Tuesday, and some will have experienced such conditions for five days by Wednesday, the Met Office’s senior meteorologist Becky Mitchell said.

To qualify as a heatwave, temperatures must meet or surpass a specific threshold for three consecutive days. The highest heatwave threshold in the UK at this time of year is 28C, which applies to London and areas north of the capital towards Cambridgeshire.

May has also had a record range in temperatures, according to Dr Stephen Burt, from the University of Reading’s department of meteorology. “From the minimum of -0.1C [air frost] recorded on 12 May, to yesterday’s maximum of 32.8C [in Reading] less than a fortnight later, a monthly range of 32.9C,” he said. The previous highest monthly range of any month was in May 1944, at 32.8C.”

The extreme weather has prompted warnings about the nation’s preparedness for the climate crisis. Friederike Otto, a professor of climate science at Imperial College London, said: “This record-breaking heat has the fingerprints of climate change all over it. Temperatures on this scale were once exceptional even at the height of summer. Seeing 35C in the UK during spring is absolutely astonishing, but the science is very clear – climate change makes these heatwaves hotter, longer and far more frequent.

“The climate we are living in today is simply not the one we grew up with, and our buildings and infrastructure are woefully unprepared for what’s next.”

Temperatures are forecast to start to decline from the middle of the week, but it is still expected to be largely dry with sunny spells. Many areas are likely to continue to experience temperatures in the high 20s. Temperatures in eastern areas, however, are forecast to be lower by about 10C as a brisk easterly wind develops.

If validated, the latest May record means highest ever temperatures have been set for seven months of the year since 2003.

A previous Met Office study found breaking the May record was “around three times more likely now in our current climate than it would have been in a natural climate not impacted by greenhouse gas emissions”. This meant that something once judged a one-in-a-100-year event was now a one-in-33-year event, it said.