Italian authorities closed roads and warned locals to be on high alert amid fears that part of a huge glacier on the Mont Blanc massif could collapse as a result of global warming.

Experts said that a quarter of a million cubic metres of ice were at risk of breaking away from the Planpincieux glacier, on the Italian side of the Alps.

Mountain refuges used by hikers and climbers were evacuated, as scientists said that the glacier was shifting at a record rate of up to 50cm (nearly 20in) a day as a result of an unusually hot summer and early autumn.

The mayor of Courmayeur, the nearest town and a popular skiing resort, ordered the closure of roads in the Val Ferret as a precautionary measure. Villages were not at risk, he said, just a few holiday chalets.

"There are no models to tell us if it will fall entirely or in pieces. We need to keep an eye on the monitoring,” said Stefano Miserocchi, the mayor.