People who suffer from hayfever have been warned to stay away from five types of tree - as the tree pollen season this year is particularly bad. An estimated 13 million people in the UK suffer from hay fever - the allergy to pollen.

The Met Office says Tree pollen occurs first, typically from late March to mid-May, and affects around 25% of people. Most people are allergic to grass pollen (which actually has two peaks) and the season lasts from mid-May until July. Weed pollen can be released at any time but the season typically covers the end of June to September.

Scientists at the University of Worcester are predicting that high levels will continue for several weeks - and the trees most likely to give it off are Birch, Oak, Cedar, Alder, and Hazel.

Dr Beverley Adams-Groom, senior palynologist and pollen forecaster, said: “Trees that produce allergenic pollen in the UK tend to have alternating high and low years of severity – this year is a high year.”

High temperatures during last spring and early summer meant there was the right weather for pollen production, with a warm May being a major factor. Dr Adams-Groom added: “Mild, dry weather with breezes provides the best conditions for pollen release and that is what is forecast for the next week or so.”

Day-to-day variations in weather can make a big difference to how much pollen is produced and how effectively it spreads. Rain can suppress pollen whereas warmth and sunshine tend to boost it - although on the very hottest days in summer levels can actually decrease.

Dry, mild, breezy days can bring the highest pollen counts. On the windiest days, pollen can be blown across a wide area meaning lower concentrations in any particular place.

The impact of climate change on pollen is likely to be mixed with considerable variations between species and in different parts of the UK.

Research by the University of Worcester found that the birch tree pollen season is getting more severe, while grass pollen continues to fluctuate around the long-term average. Most people are allergic to grass pollen (which actually has two peaks) and the season lasts from mid-May until July. Weed pollen can be released at any time but the season typically covers the end of June to September.

The Met Office said: “There are also loads of factors that change the start date of the pollen season. Low temperatures in winter will keep plants and trees dormant for longer into the new year. Essentially, the lower the temperature the less pollen is produced, but, this can change if soil and air temperatures in spring are higher than normal.

“Spring rainfall is also key, as a dry season reduces the amount of pollen production. Regardless of the weather, pollen is also dependent on how hardy different species are and how well they cope with a mixture of different types in one region.”