The 35.4-mile-long, £6bn Gotthard Tunnel will speed up railway connections between northern Europe and Italy and will depose Japan’s 33.5-mile-long Seikan Tunnel as the world’s longest.

For 14 years, boring machines have been tunnelling through the mountains from two directions: in the north from Erstfeld, near Lake Lucerne, and in the south from Bodio, near Switzerland’s border with Italy.

Only six feet of rock now separate the two tunnelling teams and the final breakthrough is scheduled for Friday.

An estimated 850 million cubic feet of debris has been removed during the construction of the tunnel as the machines cut into the rock at the rate of 50-65ft a day, the company in charge of the project, AlpTransit Gotthard AG told the Daily Telegraph.

They performed about 60 per cent of the tunnelling work, with the rest done by blasting with explosives.

Around two-thirds of the workers are Austrian and Italian, with the rest from Germany, Turkey, Spain, Portugal and the Balkans. Only a few are Swiss.

Once the excavation is completed, work will begin on laying down railway tracks, signals and safety systems.

There will be emergency stop stations every 12.5 miles where passengers could be evacuated in the event of a train crash or fire.

The tunnel is scheduled to open in December 2017, with trains travelling at 155mph, shortening the travelling time between Milan and Zurich by one hour.