Switzerland’s national rail operator has said that the world’s longest rail tunnel will not be fully reopened to train traffic until next September, again pushing back the target date for repairs of damage caused by a cargo train derailment.

The Swiss federal railway operator, known by its German-language acronym SBB, said the damage from the derailment on August 10 in the Gotthard tunnel, Switzerland’s main north-south rail thoroughfare, is “much more significant than first imagined”.

“The rail tracks need to be entirely replaced over seven kilometres (4.2 miles). The work will take much more time than anticipated,” SBB said.

The cost of the repairs is expected to be 100 million to 130 million Swiss francs (£90 million to £118 million).

Limited passenger and cargo train traffic through the tunnel is continuing. Previously, SBB said repair work would continue through to the end of this year.

No one was injured in the derailment, but the damage was considerable. Large swathes of track and an evacuation portal were torn up.

The tunnel is a crucial thoroughfare for goods and cargo, particularly between Germany to the north and Italy to the south.

Last year, more than two thirds of rail freight traffic through the Alps passed through the tunnel, according to the Swiss government.