Pollen counts have soared to "very high" across much of the UK, triggering a warning for millions of hay fever and asthma sufferers.
A "pollen bomb" has hit following last week's mini-heatwave and counts will remain at dangerously high levels as temperatures rise to 25C on Thursday and 27C on Friday.
Grass pollen season peaks during the first two weeks of July.
More than three million asthma sufferers are at risk of a potentially deadly attack, and they have been advised to stay indoors.
For about 18 million hay fever sufferers, this week's sunny and warm weather could be ruined by symptoms such as sneezing, coughing, runny or blocked noses, or headaches.
The Met Office said Wednesday's pollen levels are "very high" across England and Wales, and "high" or "medium" in parts of Scotland.
Northern Ireland's pollen count is "medium".
The conditions will affect people in rural areas and cities, where hay fever is made worse by pollution.
Temperatures will soar into the mid to high 20s in southern England and Wales this week with long spells of sunshine for some places.
This year's hay fever season started three weeks early thanks to an unusually warm winter that brought record-breaking highs of 21C in February and spring temperatures that were ten degrees warmer than usual.
Tree pollen, which affects one in four hay fever sufferers, was released earlier than usual. The season normally runs from late March to the middle of May.
Grass pollen season has two peaks between mid-May and mid-July (the second occurs in the first two weeks of July), and weed pollen season covers the end of June to September.
When grass pollen levels are high, more people are admitted to hospital with asthma attacks, said Asthma UK.
About 90 per cent of people with hay fever are affected by grass pollen.
Symptoms include sneezing and coughing, a runny or blocked nose itchy, red or watery eyes, an itchy throat, mouth, nose and ears, loss of smell, pain around the temples and forehead, headaches, earaches, and tiredness.
Hay fever is a problem for about 80 per cent of people with asthma.
People who suffer from hay fever should avoid alcohol, as it is known to worsen their symptoms.
Asthma UK suggests sufferers take their prescribed preventer medicine to soothe irritated airways so they are less likely to react to the pollen trigger.
Hay fever medicines such as antihistamines can stop the allergic reaction that triggers asthma symptoms and keep itchy eyes and runny noses at bay.
Asthma UK says: "Everyone with asthma should keep their blue reliever inhaler with them at all times in case of an emergency."
