They observed impaired foraging behaviour and increased mortality among worker bees, leading to increased likelihood of colony failure.
Gill said: "The novelty of this study is that we show how the sub-lethal effects of pesticide exposure affects individual bee behaviour with serious knock-on consequences for the performance of the colony as a whole."
Raine added: "Bees that were exposed to both pesticides consistently performed most poorly in all our measures. Currently pesticide usage is approved based on tests looking at single pesticides, but our evidence shows that the risk of exposure to multiple pesticides needs to be considered, as this can seriously affect colony success."
The findings, published in the journal Nature, follow on from two other studies in Science earlier this year claiming to show a connection between neonicotinoids and bee decline. Defra has said these studies "do not justify changing existing regulation", while the industry-funded Crop Protection Association accused these earlier studies of being "unrealistic".
Publication of this latest study also coincides with the European Food Safety Authority's public consultation on the risk assessment of plant protection products on bees, which closes for submissions on Thursday (25 October).