One of America's most irritating weeds threatens to spoil the summer months of thousands of Britons who are prone to crippling hay-fever attacks. Ragweed has established a beachhead in central Europe and is spreading westwards towards Britain, scientists said yesterday.

A single ragweed plant can spew out a billion, highly allergenic pollen grains in just one season and this highly oversexed, invasive US weed is now firmly over here on this side of the Atlantic, they warned.

An increasing number of Europeans are showing signs of ragweed allergy as the plants spread from Hungary to the fields of Italy, Austria, France and more northerly regions bordering the English Channel. Scientists fear it might only be a matter of time before the common ragweed, Ambrosia artemisiifolia, gains a foothold in Britain where it could become an invasive species with the help of warmer summers, milder winters and the formidable reproductive powers of the plant itself.

A scientific conference in Vienna this week has been called to address the problems posed by the plant species, which spreads with frightening rapidity along roadside verges, railway lines and newly cleared land.

"Common ragweed is not an issue here in the UK – yet. As global temperatures rise we are seeing very rapid spread of this highly invasive plant and it may only be a matter of time before it appears in the UK," said Dr Clare Goodess, of the Climatic Research Unit at the University of East Anglia. Common ragweed, which grows to a height of about a metre, is particularly irritating to the human immune system, producing pollen grains that are highly allergenic.

Warmer summers extend the pollen season and higher concentrations of carbon dioxide in the atmosphere have been shown to boost the production of ragweed pollen.

In the US, ragweed pollen is one of the most common causes of hay-fever and asthmatic attacks – 75 per cent of Americans who are allergic to pollen are allergic to ragweed pollen. It can also travel long distances, being found 400 miles out to sea and two miles up in the atmosphere – although the highest concentrations are found close to where the plants flower in late summer.

The northern limit of ragweed in Europe is moving further north with climate change, according to Dr Jonathan Storkey, a plant ecologist at Rothamsted Research in Harpenden, Hertfordshire. "The English Channel won't be a barrier to it," he said.

"The concern about ragweed centres on the health issue rather than the problem of its invasiveness. We have invasive plant species already but this species has pollen that is highly allergenic – it's bad news for hay-fever sufferers."

Like other types of hay-fever allergies, ragweed symptoms include a runny nose, sneezing, puffy or irritated eyes, and a stuffy or itchy nose and throat. Because ragweed produces its pollen between late summer and the first autumn frosts, it can significantly extend the hay-fever season.

Tests by European scientists published two years ago showed that up to 60 per cent of people in Hungary show allergic sensitivity to ragweed pollen. High rates of sensitivity were also found in Denmark, the Netherlands and Germany and about a quarter of those showing ragweed pollen allergy also had symptoms of asthma.

Professor Torsten Zuberbier, of the Charite University of Medicine in Berlin said that ragweed pollen sensitivity was currently affecting about 2.5 per cent of the wider European population, which is the current threshold for a "high prevalence" allergy.

"The study highlights the spreading of [ragweed] pollen and the dissemination of the plant throughout Europe," Professor Zuberbier said.

The University of East Anglia with use is climate models to assess how rapidly ragweed is likely to spread further north in Europe given that the plant requires a long, hot summer to produce its pollen. Ian Lake, a climate modeller at East Anglia, said: "We will analyse the likely impact of changes in climate, land use and air pollution on pollen-induced allergy over the coming decades and devise adaptation and prevention strategies to minimise the impact on global health."

American ragweed has been present in Europe for about a century but it emerged as a seriously invasive plant species in Hungary during the 1990s.

Plant strife: invasive species from abroad

Rhododendron

A native of the Himalayas, they have spread from gardens to wild areas, where they shade out native plants.

Water primrose

Can cause havoc to waterways when introduced in the wild.

Japanese knotweed

A pernicious weed that can propagate from tiny fragments. Any soil contaminated has to be disposed of at registered sites.

American skunk-cabbage