Almost a quarter of European bumblebee species are threatened with extinction, largely because of climate change and intensive farming, the International Union for Conservation of Nature warned on Wednesday.

In a European Union-funded study of all 68 bumblebee species found in Europe, the environmental group found that 24 percent were on the verge of disappearing.

The research also found that 46 percent had a declining population, including those at risk of extinction, while 29 percent were stable, 13 percent were increasing in number and the status of the remainder was unknown.

Like the collapse in numbers of other bee species, the loss of bumblebees is a threat to humans because they play a crucial role in pollinating plants, and are thus a key link in the food chain.

Of the five most important pollinators of European crops, three are bumblebee species, the IUCN said.

The main threat to bumblebees are climate change, the intensification of farming and changes in the use of agricultural land, the IUCN said.

It also cited pollution from agricultural waste and loss of habitat due to urban development.

"We are very concerned with these findings. Such a high proportion of threatened bumblebees can have serious implications for our food production," said Ana Nieto, the IUCN's European biodiversity officer, who coordinated the study.

"Protecting bumblebee species and habitats, restoring degraded ecosystems and promoting biodiversity-friendly agricultural practices will be essential to reverse the negative trends in European bumblebee populations."

Among the species hit hardest by climate change is the Bombus hyperboreus -- Europe's second-largest bumblebee -- whose habitat in the Scandinavian tundra and the extreme north of Russia is shrinking fast.

The Bombus cullumanus, meanwhile, has seen an 80-percent population decline in the past decade as farmers strip away clover, its main forage.

Previously widespread, it is now only found in a handful of scattered locations across Europe.

And Europe's largest bumblebee, the Bombus fragrans, is also seriously threatened by the intensification of agriculture in its native habitat in the steppes of Ukraine and Russia.

"Many of these species live in very restricted areas and in low numbers," said Pierre Rasmont of the IUCN's bumblebee unit.

"They are often extremely specialised on their host plants, which makes them susceptible to any environmental change."