The world’s longest rail tunnel has been shut to passenger services after a derailment in Switzerland causing damage that will take months to repair.

Swiss authorities said there was “no indication” when the Gotthard Base Tunnel would reopen.

Sixteen carriages derailed and are still stuck inside, a week after the derailment on 10 August.

Opened in 2016, the Gotthard rail link took 20 years to build and cost more than $12bn (£8.2bn).

National Swiss rail operator SBB chief executive Vincent Ducrot said on Wednesday that Gotthard was one of the safest tunnels in the world.

“The fact that an accident like this could happen has hit us very hard,” he said. “Fortunately, there were no injuries, though significant material damage was incurred.”

An investigation is being led by the Swiss Transportation Safety Investigation Board (STSB) and the public prosecutor’s office of the Canton of Ticino.

Rail operator SBB said in total, around 8km (4.9 miles) of track and 20,000 concrete sleepers need to be replaced.

SBB said one side of the tunnel was unaffected and should be operating “in principle” for freight usage from 23 August, but passenger trains would not be able to use the unaffected side due to safety reasons.

In a statement SBB said: “Since the derailment, the tunnel has been closed to passenger and freight traffic, and trains must be diverted.

“Currently, there is no indication of how long the tunnel will be closed.

“Diverting trains via the panorama route means longer journey times for passengers.”

The route cuts through the Alps and provides a high-speed link between northern and southern Europe.

As well as transporting passengers, Gotthard is a key passage for cargo – particularly between Germany and Italy.

More than two-thirds of rail freight traffic in the Alps going through the tunnel last year.

Journey times will now take at least 60 minutes longer and passengers travelling internationally will need to change trains in Chiasso.

Only trains travelling to and from Genoa and Venice will run directly, SBB said.