LONDON, July 23 (Reuters) - Gardens provide a lifeline for Britain's dwindling bumblebee population and are a far more popular nesting location than open countryside, scientists said on Monday.

The plight of the bumblebee prompted a nationwide survey by conservationists, involving more than 700 volunteers, which found Britons' popular back gardens contained the highest densities of bumblebee nests at 36 per hectare.

That was two to three times higher than grassland and woodland.

Until now, little has been known about which habitats are best for the furry yellow-and-black insects and experts hope the finding will aid conservation work.

Britain once had more than 25 native species of bumblebee, but three of those have been lost in the past 50 years and several more are under threat. Scientists blame the decline on modern farming methods that have deprived bumblebees of many traditional flowering plant food sources.

Gardens, by contrast, have plenty of flowers and good nesting sites -- particularly messy ones, according to lead researcher Juliet Osborne from the Rothamsted Research centre in central England. Favourite nesting spots include undisturbed flower beds and compost heaps.

Osbourne's research was published in the Journal of Applied Ecology.