A block of 250,000 cubic metres of ice could break away from the Planpincieux glacier, on the Grandes Jorasses mountain of the Mont Blanc massif, officials said.
“These phenomena once again show how the mountain is going through a period of major change due to climate factors and, therefore, it is particularly vulnerable,” said Stefano Miserocchi, mayor of the nearby town of Courmayeur.
Miserocchi signed an order closing roads in Val Ferret after experts warned that the glacier's movement had accelerated 50-60cm per day.
He said there was no threat to residential areas or tourist resorts, but “unoccupied mountain huts” in the area were being evacuated as a precaution.
Experts at the Fondazione Montagna Sicura (Safe Mountain Foundation), who have been monitoring the glacier for the Val d'Aosta region since 2013, alerted local officials to the latest developments.
“This glacier is atypical because it's temperate, and so is influenced by the temperature of the water flowing below, which particularly exposes it to the global warming in progress,” said the foundation's secretary-general Jean Pierre Fosson.
But he cautioned against alarmism, stressing that the preventive measures taken so far were for an “unprecedented situation” for a glacier in the region.
While it might break off in a single block, it could just crumble away or not break away at all, he added.
The Foundation monitors 180 glaciers in the Val d'Aosta region and this kind of thing is unavoidable, said Fosson.
“Every year we see two square kilometres (0.8 square miles) of ice disappear” he said. “And it is getting worse with the increasingly hot summers and autumns.”
Experts say rapidly retreating glaciers, an important source of fresh water, are among the many symptoms of the man-made climate crisis.
According to a report published this week by the World Meteorological Organisation, if temperatures continue to rise at current levels the ice on the Eastern and Central Alps could be drastically reduced or even disappear completely within two or three decades. Ice would remain only in the Western Alps, which are the highest.