Between 40,000 and 50,000 bumblebee colonies are imported into England each year to assist with crop pollination.

For a study in the Journal of Applied Ecology, scientists bought 48 colonies - hives containing up to 100 bees each - from three producers in Europe.

They found 77% had parasites that could infect native bees.

Lead researcher Prof William Hughes, of the University of Sussex, said commercial production and importation of bumblebees had been "going on for decades".

"We couldn't grow tomatoes in this country without these bumblebees," he said.

And with the decline in pollinating insects in recent years, food producers are increasingly reliant upon imported bees.

"Over a million colonies are imported globally - it's a huge trade," said Prof Hughes. "And a surprisingly large number of these are produced in factories, mainly in Eastern Europe.

"We sought to answer the big question of whether colonies that are being produced now have parasites and, if so, whether those parasites are actually infectious or harmful."

For more information got to:http://www.bbc.co.uk/news/science-environment-23347867