The double impact of decline in bee numbers and global warming is not only cutting down plant pollination but will also badly impact global food output.

In fact, pollination levels of some plants may have steeply dropped by 50 percent over the last two decades, which could slash crop yields.

The research, carried out in the Rocky mountains of Colorado, is the first to show that the effect is real, reports the Telegraph.

It serves as 'warning' to Britain, which has seen an even greater decline in bees and pollinators.

'This serves as a warning to other countries,' said James Thomson at the University of Toronto, Canada, who carried out the research.

The findings are scheduled for publication in the journal Philosophical Transactions of the Royal Society B: Biological Sciences.

According to a previous study, England's bees are vanishing faster than elsewhere in Europe, with more than half of hives dying out over the last 20 years.

Butterflies and other insects are also in decline due to habitat loss and climate change.

Thomson carried out a 17-year examination of the wild lily in the Rocky mountains. It is one of the longest-term studies of pollination ever done.

It reveals a progressive decline in pollination over the years, with particularly noteworthy pollination deficits early in the season.

Thomson began his long-term studies in the late 1980s after purchasing a remote plot of land and building a log cabin in the middle of a meadow full of glacier lilies.