The nationwide failure of its fixed-line network was the latest by the state-controlled company and followed three similar problems in 2020.

In the latest case, part-time firefighters were called up to their stations to help in emergencies when the entire fixed-line phone network for Switzerland failed in the early hours of Friday morning.

"I apologise to the firefighters and everyone involved. The failure shook me a lot. That is absolutely not what we expect from ourselves at Swisscom," Schaeppi told the Neue Zuercher Zeitung in an article published on Wednesday.

The collapse was caused by a software malfunction when the company was carrying out maintenance work on its telephony platform for business customers, he said.

"A software update led to a malfunction that triggered a domino effect," Schaeppi said. "Although stability is our top priority, individual failures can occur. Unfortunately, this cannot be prevented," he told the newspaper.

(Reporting by John Revill; Editing by Stephen Coates)