Construction works could begin in 2018 and complete in 2021. Those planning to use the tunnel in the coming months should count on occasional closures due to geological surveying.

The single tube, two-lane tunnel linking the towns of Jesenice in northwestern Slovenia and Villach in Austria opened in 1991 and quickly became one of the key transit points between the two countries.

The tunnel sees very heavy traffic during holiday season, especially in the summer, when kilometres-long jams form on both sides.

According to the most recent plan, the new tube would open exactly 30 years after the first tube. However, both would not be operational until 2023, as the old tube would have to be renovated before both are opened for one-way traffic.

Slovenian Motorway Company DARS will launch a geological study of the terrain on Monday. Works will take place only on the Slovenian side of the tunnel, as Austria has already carried out the required surveying studies in 2014 and 2015.

Geologists and hydrologists will not be drilling only into the mountain slopes above the tunnel, but also through the walls of the existing tube and the tunnel will be closed in the direction of Austria several times during the day.

Closures are scheduled between 7 AM and 7:30 AM, 3 PM and 3:30 PM, and 11 PM and 11:30 PM. Moreover, the tunnel will be completely closed for traffic for periods of up to an hour between 1 AM and 4 AM.

The closures are scheduled until May, however not during the Easter holidays, when heavy traffic is expected.

Under the bilateral agreement, Slovenia is to build 3.5 kilometres of the tube and a half-kilometre slipway, estimated at nearly EUR 150m excluding tax. The government is currently working on a zoning plan for the area, which is to be adopted in June.