Health experts are warning that a toxic haze will cover much of the UK after the highest ever UK temperature in May was recorded.
Searing highs will cause air pollutants to rise, leading to 'ground ozone' forming and affecting parts of the country until Saturday.
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People with breathing difficulties are being urged to stay indoors during the hottest periods.
The alert was issued after the highest UK temperature ever recorded in May – 26.6C – was clocked at Pershore in Worcestershire on Wednesday, the Birmingham Mail reports.
Weather air pollution and smog covers the Liverpool waterfront skyline
Central London covered in smog in a photo from 2015, as Britain prepares for a toxic haze
But the Met Office said today's maximum in the West Midlands could be even higher, with a forecast of 27C.
The Met Office forecast temperatures of 25C in Birmingham on both Thursday and Friday, before a cold front from the west sends temperatures tumbling on Sunday and Monday.
But experts from the Department for Environment, Food and Rural Affairs advised that 'moderate' pollution would affect western Britain on Thursday before spreading across the country on Friday and Saturday.
People enjoying the sunshine and warm weather on Brighton beach in East Sussex
People sunbathing as a heatwave hits the country
A woman holds a parasol as she walks through the grounds of the Chelsea Flower Show
A spokesman said: "Air pollution levels should remain low across many parts through Thursday, though some areas of moderate pollution are likely mainly in central and western parts.
"With high pressure across the country areas of moderate pollution are likely until Sunday."
The toxic haze will first affect Wales, the north-west and central regions on Thursday before spreading to Scotland, western Britain and parts of the south on Friday.
The rest of the UK is likely to be covered by Saturday.
Dr Penny Woods, chief executive of the British Lung Foundation, urged people with breathing difficulties to stay indoors during the hottest periods.
Haze and fog covers Newcastle
She said: "During very warm weather, people living with lung conditions may find high temperatures mean their symptoms get worse.
"This can be because they're too hot, dehydrated, or because of high ozone levels in the air."
Near the ground, ozone is made by a chemical reaction between the sun's rays and organic gases and oxides of nitrogen, emitted by cars, power and chemical plants, and other sources.
She added: "For people with a lung condition, high ozone levels can make breathing difficult, causing wheezing and coughing.
"If you have a lung condition, we advise keeping out of the sun and avoiding the heat."
Kay Boycott, chief executive of Asthma UK, added: "Our research tells us that two thirds of people with asthma find that air pollution makes their asthma worse, putting them at increased risk of a potentially life-threatening asthma attack."