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Britons are being urged to do the "neighbourly thing" and check on people who may be more vulnerable as the UK braces for potentially record-breaking temperatures.
The advice comes after the government's COBRA meeting on Saturday - which was not attended by the outgoing prime minister - and less than 48 hours away from forecast highs that could nudge 40C (104F).
It's the second emergency summit held since the UK Health Security Agency increased its heat health warning from level three to level four - a national emergency.
Cabinet Office minister Kit Malthouse, who chaired the meeting, said older people and babies were "particularly vulnerable groups, who will need us all to look out for them".
"[People] should take precautions themselves - stay in the shade, drink lots of water, stay hydrated," he said.
"But in particular, they should look out for those two groups and do the neighbourly thing… knock on an elderly neighbour's door, make sure they are okay, that they've got access to water.
"If you've got a tiny baby, make sure it's kept cool, possibly in a colder bath or chilled in some way."
There's now an 80% chance that temperatures will top the UK's record of 38.7C (101.7F), set in Cambridge in 2019, according to forecasters, with a 50% chance of 40C on Tuesday.
The hottest area - under a Met Office "extreme heat" alert on Monday and Tuesday - covers all the way from London in the south, through the Midlands and as far north as Greater Manchester and York.
The rest of England and Wales is under an amber warning, while Scotland and Northern Ireland will be much cooler, with temperatures in the mid to late 20s.
People warned not to swim in lakes or rivers
Fire and rescue services and water service companies that operate reservoirs have issued warnings to those wanting to swim outside during the heatwave.
Avon Fire and Rescue has warned against swimming in lakes, ponds or rivers, while Bristol Water has warned about "strong underwater currents" at its Cheddar Reservoir.
It comes as West Yorkshire Police were called to a report for a concern for safety on Saturday in relation to a person in the water at Ardsley Reservoir.
The force said emergency services were currently at the scene and a search was under way.
Elsewhere, a body has been located in Salford Quays after Greater Manchester Police received a report of a person getting into difficulty in the water on Saturday evening.
Railways and schools brace for extreme heat
Meanwhile, train companies have warned of disruption and cancellations when the heat peaks due to factors such as reduced speed limits.
Network Rail said people should only to travel if necessary.
"Services are going to be significantly affected. The heat will impact rails, for example, so the trains have to run slower. There may be fewer services. People need to be on their guard for disruption," said Mr Malthouse.
"If they don't have to travel, this may be a moment to work from home," he added.
Drivers have also been advised to make their journeys out of the hottest periods of the day, particularly if they have older cars.
Some schools are also planning to shut due to risks such as dangerously hot classrooms. The NEU teaching union has said it will support headteachers taking this decision.
The government is not imposing closures and has said it's up to school leaders to decide what action to take.
Some schools have said they will arrange a skeleton staff to keep the buildings open for parents unable to find alternative childcare at short notice.
Nurseries in some locations will also be restricting their hours following Public Health England's guidelines for supporting children in Early Years.
The majority of UK schools are set to break up for the summer holidays later in the week.