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The April heatwave is about to come to an end but the hay fever misery that came with it will continue, experts say.

Weather systems that have been pulling in warm winds up from the equator are being displaced by Atlantic depressions which will trigger a temperature change.

But a nasty "pollen bomb" will be left behind, caused by simultaneous releases from millions of birch, plane and oak trees.

Millions of people in southern England are already suffering but it is set to spread northwards in the next few days.

Guy Barter, chief horticulturist at the Royal Horticultural Society, said: "Tree species normally release pollen at different times in spring, starting with willow and alder in February and March.

"These are wind-pollinated species, which make vast amounts of pollen - one birch or hazel catkin makes 5m grains, and a tree has thousands of catkins. We are swimming in clouds of pollen - we just can't see it."

The National Pollen and Aerobiology Research Unit at Worcester University, which produces pollen forecasts, says levels will peak over the weekend before only a slight decline going into next week.

Mr Barter said: "The tree pollen count will decline soon but then we face an even worse situation in May when grass starts to flower.

"Grass produces far more pollen simply because there is so much of it."

The medical term for hay fever is allergic rhinitis. It is triggered when our immune systems mistake airborne particles such as pollen for a harmful pathogen which usually causes inflammation of the airways.

Pollen expert Shenagh Hume told Sky News that sometimes people mistake hay fever for a spring cold.

"They sneeze or get bunged up and think it's just a cold. But if your eyes are itchy or sore, you are sneezing or bunged up at this time of year, it is likely to be hay fever.

"Hay fever can be serious, especially for people with asthma because the size of the airways is reduced by pollen."

She said around 40% of the UK population suffers from hay fever.