"The majority of the fungus is an underground network that looks a bit like shoelaces. The surface mushrooms look like the normal type you would pick, and are brown to yellow," said Muriel Bendel, a spokeswoman for the Swiss research association for forestry, snow and countryside (WSL).

The fungus, "Armillaria ostoyae" or honey mushroom, is edible, the WSL said, adding it had been known since Roman times for its cleansing effects on the digestive tract - as long as it was eaten raw. But certain forms can kill trees.

The WSL said Switzerland's monster mushroom was trumped only by a growth in the United States which covers a surface area of nine sq km and weighs an estimated 600 tonnes.