Fears have been mounting over Russian sabotage after a critical underwater internet cable was severed.

The 745-mile fibre optic cable running through the Baltic Sea from Helsinki to the German port of Rostock may have been severed by an outside force, a Finnish state-controlled cyber security and telecoms network company said.

The C-Lion1 cable malfunctioned just after 2am on Monday, firm Cinia said.

Finland and Germany said in a joint statement that they were ‘deeply concerned about the severed undersea cable’ and were investigating ‘an incident (that) immediately raises suspicions of intentional damage’.

Europe’s security is threatened by Russia’s war against Ukraine and ‘hybrid warfare by malicious actors,’ the joint statement said, without naming the actors.

‘Safeguarding our shared critical infrastructure is vital to our security and the resilience of our societies,’ Germany and Finland said.

The sudden outage implied that the cable was completely severed by an outside force, although a physical inspection has not yet been conducted, Cinia’s chief executive, Ari-Jussi Knaapila, told a press conference.

The damage to the Finland-Germany cable occurred near the southern tip of Sweden’s Oland Island and could require five to 15 days to repair, Knaapila said.

Cinia said that most internet users would not notice the outage and that it was likely a result of human activity, although they added that there was no indication that the damage was the result of sabotage.

‘At the moment, there is no way to assess the cause of the cable break, but such breaks without external impact do not happen in these waters,’ Knapila said.

He added that a trawler or a ship putting down an anchor due to an emergency could have severed the cable, which is covered in a double-armoured steel casing.

Swedish public service broadcaster SVT reported that Swedish authorities were also investigating damage to a communications cable running between Lithuania and Sweden, close to the one that was severed.

‘It is absolutely central that it is clarified why we currently have two cables in the Baltic Sea that are not working,’ Carl-Oskar Bohlin, minister of civil defence, told SVT.

The episode recalled other incidents in the same waterway that authorities have probed as potentially malicious.

Last year, a subsea gas pipeline and several telecoms cables running along the bottom of the Baltic Sea were severely damaged in an incident raising alarm bells in the region.

Investigators of the 2023 cases in Finland and Estonia have named a Chinese container ship that they believe dragged its anchor and caused the damage.

But they have not said whether the damage was accidental or intentional.

In 2022 the Nord Stream gas pipelines linking Russia to Germany in the Baltic Sea were destroyed by explosions in a case that remains under investigation by German authorities.

Located in northern Europe, the Baltic Sea is an active commercial shipping route and is ringed by nine countries including Russia.