The 57km (35-mile) twin-bore Gotthard tunnel will provide a high-speed rail link under the Swiss Alps between northern and southern Europe.

Switzerland says it will revolutionise European freight transport.

Goods currently carried on the route by a million lorries a year will go by train instead.

The tunnel will overtake Japan's 53.9 km Seikan rail tunnel as the longest in the world and push the 50.5 km Channel Tunnel linking the UK and France into third place.

German Chancellor Angela Merkel, French President Francois Hollande and Italy's Prime Minister Matteo Renzi are due to attend the grand opening.

The project, which cost more than $12bn to build, was endorsed by Swiss voters in a referendum in 1992. Voters then backed a proposal from environmental groups to move all freight travelling through Switzerland from road to rail two years later.

The completed tunnel travels up to 2.3 km below the surface of the mountains above and through rock that reaches temperatures of 46C.

Engineers had to dig and blast through 73 different kinds of rock, some as hard as granite and others as soft as sugar. More than 28m tonnes of rock was excavated. Nine workers died during the work.

Now the completed tunnel - delivered on time and within budget - will create a mainline rail connection between Rotterdam in the Netherlands and Genoa in Italy.

Its trajectory will be flat and straight instead of winding up through the mountains like the old rail tunnel and a road tunnel opened in 1980.

About 260 freight trains and 65 passenger trains will traverse it in as little as 17 minutes every day once testing ends later this year.

The tunnel is being financed by value-added and fuel taxes, road charges on heavy vehicles and state loans that are due to be repaid within a decade.

Swiss bank Credit Suisse has said is economic benefits will include the easier movement of goods and increased tourism.

-BBC