The Met Office has issued its first ever extreme heat warning for the UK with temperatures possibly reaching 33C in western areas.

The amber warning is in place for much of Wales, all of south-west England and parts of southern and central England.

It also warned of increasing water safety concerns as six people died in incidents over the weekend.

“Many areas will continue to reach heatwave thresholds but the amber extreme heat warning focuses on western areas, where the most unusually high temperatures are likely to persist,” said the Met Office’s chief operational meteorologist, Steven Ramsdale.

An amber warning is the second-highest level under the Met Office’s new extreme heat warning service launched in June. It said temperatures in most areas covered by the warning would be in the high 20s and low 30s.

A note accompanying the alert warned of health effects on vulnerable people as well as heat exhaustion and sunburn for the wider population.

“I wrote to the prime minister last year and again at the start of this summer to warn that the country was not properly prepared for the growing risks from hot weather and needed a national heat risk strategy. We know that heatwaves are becoming more intense and frequent because of climate change,” said Bob Ward of the LSE’s Grantham research institute on climate change and the environment.

“We are now facing conditions that are similar to the period of hot weather that occurred in August 2020, resulting in more than 1,700 deaths across England.”

He said elderly people and those with respiratory illnesses were most vulnerable but deaths could be avoided with a national plan to manage the increasing heat.

The Met Office also said the likelihood that people would flock to coasts, rivers and lakes would increase water safety concerns. At least six people died in such incidents over the weekend, leading to calls from emergency services warning people against swimming in open water.

Derbyshire fire and rescue service said a man’s body was recovered after a four-hour search on Sunday night at Victory Quarry near Buxton. They added a warning that the cold water makes it difficult even for accomplished swimmers.

South Yorkshire fire and rescue issued a similar appeal after a man in his 20s was recovered from the 30ft deep lake in Sheffield’s Crookes Valley Park before midnight on Sunday.

“Our message to people across South Yorkshire is simple – please stay out of the water unless you are at a proper swimming pool or part of an organised open water swimming group,” said the statement.

“Open bodies of water are often much colder than they look – this can send your body into shock when you jump in and prevent you being able to swim to safety. You also have no idea what is underneath – hidden currents, rocks and rubbish being the main dangers that can cause you serious harm.”

Elsewhere a 19-year-old man died after going into the water in Salford Quays, Greater Manchester, and a teenage girl was pronounced dead after being retrieved from Ducklington Lake in Oxfordshire.

North Yorkshire police said a man in his 50s was pronounced dead on the River Ouse. Dorset police said a man in his 40s fell from rocks near Stair Hole on the Jurassic Coast. He was pronounced dead at the scene.

The Met Office also said there could be an increased risk of wildfires as well as damage to heat-sensitive equipment and potential power cuts.

The West Midlands fire service put out a fire when a bus stop self-combusted in Chelmsley Wood suburb of Solihull.