Massive landslides on a Herculean scale are threatening to destroy a picturesque hamlet above the popular ski resort of Courmayeur and bury the approach road at the Italian end of the Mont Blanc Tunnel under nearly two million tons of granite.

Already in recent days, traffic through the tunnel has been halted for up to three hours at time because of giant boulders tumbling down the mountainside.

The tunnel could remain blocked for a considerable period until a new road is built. After the fire that killed 38 people in 1999, the tunnel was closed for three years. This resulted in traffic chaos across a substantial part of the Alps. As well as being a popular tourist route, the tunnel is one of the main freight arteries of the continent.

Italian engineers have been monitoring the slippage on the Mont de la Saxe 24 hours a day since 2009. But in the current warm spring weather, the cracks are widening at a rate of four metres a day and the first of what could be a two-pronged disaster appears to be imminent.

Some 80 residents in the hamlet of La Palud have been evacuated and all mountain roads sealed off. As one of the scientists monitoring the cracks said yesterday: “It appears that now the slide is no longer a question of ‘if’, but when.”

To give you an idea of the scale of the impending natural disaster, the first major rockfall is expected to be at least 300m wide by 45m deep – a similar mass of granite to the concrete and glass of the twin towers of the World Trade Centre that fell on Manhattan. Earthquake centres around the world are expected to record the seismic wave.

Apart from flattening La Palud and destroying the tunnel approach road, the landslide is likely to block a river swollen with snow melt, and this could cause flooding further down the Aosta Valley. But the real fear is that the slide will trigger a second rockfall higher up the mountain that could be more than 12 times bigger.

At the moment the situation is like a real life version of Jenga, the table-top game in which you build a tower of 54 wooden building blocks and each player removes one at a time from the middle. No one knows until the first landslide has taken place, whether the higher part of the mountain will remain as a giant overhang, or come tumbling down – all 24 million tons of it.

Apart from tunnel access, Courmayeur and its main ski area are sufficiently distant to be entirely unaffected by the looming disaster.

The Italian government gave approval yesterday for work to start on a £6.5 million defensive barrier, but residents evacuated from La Palud for the second time in a year described the action as “too little, too late.”

You might also be interested in reading our feature, Avalanche: A surviver’s Guide.