Dr Peter Graystock of the University of Leeds said: "We found that commercially-produced bumblebee colonies contained a variety of microbial parasites, which were infectious and harmful not only to other bumblebees but also to honeybees."
His colleague Professor William Hughes of the University of Sussex added: "Many bee species are already showing significant population declines. The introduction of more or new parasite infections will at a minimum exacerbate this and could quite possibly directly drive declines."
The researchers estimate that 40,000-50,000 commercially produced bumblebee colonies, each containing up to 100 worker bees, are imported annually to the UK for crop pollination - part of a worldwide trade of more than one-million colonies each year.
The findings appear in the Journal of Applied Ecology, published by the British Ecological Society.
New regulations introduced at the start of this year already require growers of protected crops to register their premises with Natural England before releasing imported bees. These also require "all reasonable steps" to be taken to prevent their escape. Unlicensed release of non-native bumblebees can lead to prosecution.
But imported hives are not screened for disease and release of native subspecies of Bombus terrestris is not covered by the regulations.
Dutch beehive supplier Koppert Biological Systems, which was not involved in the study, said the findings "emphasise the importance of effective monitoring" and added: "Our bumblebee colonies are produced in a protected environment under extremely highly controlled conditions. The entire production process is certified."