Hot air is set to move in from south-west Europe, as forecasters warn sunbathers to “avoid overdoing it”.
Parts of England and Wales are set to swelter this week with Friday likely to be the hottest day of 2022 so far.
Temperatures are forecast to top 30C (86F) in the south-east of England as hot air spreads up from Iberia, in the south-western corner of Europe.
The Met Office has warned sunbathers to “avoid overdoing it” and stay out of the sun in the midday hours.
But northern parts of the UK will not be as hot, with temperatures expected to rise only to the mid-teens.
“[It will be] the first hot spell of weather of the year for some parts of the country,” said the Met Office’s Alex Deakin. “But quite a contrast, it’s not going to be hot and sunny everywhere.”
He said temperatures will be “ticking up day on day” from Wednesday, with highs of 27C (80.6F) and 28C (82.4F) expected across the South East – although just 14C (57.2F) and 15C (59F) across the north west of the UK.
The peak of the temperatures is set to be Friday, when the South East could see 32C (89.6F) or 33C (91.4F) around north London and Cambridge, with high UV levels and high pollen levels also forecast.
But temperatures would need to beat 35.6C (96.1F) to break the record for the UK’s hottest June day, which was recorded in Southampton in 1976.
It comes as parts of Europe are also experiencing extreme heat – with Spain in the grips of a heatwave as temperatures top 40C, and the hot weather set to spread to parts of southern France from Tuesday.
“For many places, Friday will be the warmest day of the year so far,” said Mr Deakin.
But the picture from Saturday is more uncertain, he said, with possible showers and cooler air.
The Met Office has not yet issued a weather warning for extreme heat, which it did for the first time ever last July.
An official heatwave is not out of the question, which is when at least three consecutive days have daily maximum temperatures meeting or exceeding the heatwave temperature threshold.
The threshold varies by county, and in the UK’s is between 25C and 28C depending on the part of the country. The threshold was raised by 1C in some areas earlier this year.
Experts say that while heatwaves are extreme weather events, scientific research shows that climate change is making these events more likely.
Met Office forecaster Craig Snell told the PA news agency: “Very high UV levels are expected in the south this week, meaning people should really avoid being in the sun during the midday hours.
“Wearing sunglasses, a shirt, a hat and sunscreen are essential to protect the eyes and skin, and drinking lots of water is important – long exposure to the sun can be dangerous.
“People should avoid overdoing it. We all love the sunny weather, but being sensible can help you avoid a nasty sunburn.”
The Royal Life Saving Society UK which promotes water safety has issued a plea to those looking to cool off in water, urging people to understand the dangers of drowning.
Lee Heard of RLSS UK said: “Ahead of the expected 30C temperatures in the coming week and with June, July, and August proving to be the months with the most fatalities, it is vitally important that everyone has an understanding of water safety, especially during the summer months.
“We have seen a rise in the number of drownings over the last few years, with peaks during the summer.”