Spring this year will be heralded by the sound of sneezing, coughing and nose blowing as the nation's 12 million hay fever sufferers make an early start to the season.

Climate change means that the trees and grasses most associated with hay fever are flowering early.

Because of warm winters, sufferers can experience symptoms as early as the end of February and the season which once ended in July runs through August.

For reasons that are not yet understood, the number of people suffering from symptoms is rising dramatically. In 1965, 12% of young people aged 12-14 suffered from hay fever, while currently it is 36% and continuing to grow. Adults in their 40s are developing hay fever and recent tests on children aged six and seven showed 25 to 30% have hay fever depending on where they live. The Midlands has most sufferers, probably because they live furthest from the sea and pollen concentration is highest.

The difference between one year and the next can be marked, depending on how warm the winter. In 2002 flowering for the most troublesome grasses was nine days to two weeks earlier than in 2001.

Last year, silver birch trees, normally the first triggers of hay fever, came into leaf on April 8 on average, compared with April 25 in 2001. What was not studied was when pollen from the catkins was released: as this is normally well before the leaves appear it could account for the early start of the hay fever season.

Most sufferers are allergic to grass pollen. In 2002 one of the troublesome species, meadow foxtail, flowered on May 5 compared with May 18 the previous year, yorkshire fog on May 28 rather than June 6, and coxsfoot on May 21 instead of May 30.

Michael Fish, Met Office meteorologist and hay fever sufferer, is appealing for volunteers to record seasonal changes, a study called phenology, which helps keep a check on the progress of climate change.

Mr Fish said: "High temperatures are extending the grass flowering season. Like me, many people will be condemned to longer bouts of sneezing, itchy eyes, headaches and throbbing nose. Phenology helps to forecast the start of the season and warn sufferers."

Professor Jean Emberlin of the National Pollen Research Unit said: "The birch pollen season has been occurring five to 10 days earlier per decade over the last 30 years.

"Forecasting when it will happen will help people to take their medication at the right time."