Switzerland was responsible for last month’s blackout that affected almost all of Italy, according to a report by European grid operators.
In its interim report, the Union for the Coordination of Transmission of Electricity (UCTE) said Swiss officials did not respond quickly enough after a power line shorted out in a storm.
The report criticised the measures taken in Switzerland as “inadequate” and “insufficient”, adding that the Swiss authorities could have avoided the blackout altogether had they responded better.
It said the Swiss underestimated the urgency of the situation and did not take effective countermeasures.
Responding to Monday's report, Etrans, coordinator of the Swiss grid, rejected the findings, saying they were "one-sided".
Swiss officials have always maintained that they rectified the situation as quickly as possible, blaming Italy's power grid for failing to react in time.
The UCTE said the power outage was triggered after a tree fell on a transmission line on the Lukmanier pass in central Switzerland.
This led to high loads on the remaining lines. Ten minutes later, Etrans called its Italian counterpart, GRTN, asking it to decrease imports of electricity to relieve the overload.
The report said Etrans lacked a sense of urgency in its handling of the matter, making only one call. The UCTE accused the Swiss of failing to realise that the system could only handle the overload for 15 minutes.
Therefore, minutes later when another line was struck by a tree on the San Bernardino pass in eastern Switzerland, the overload on the remaining lines became “intolerable”.
Following the second incident, all the other inter-connectors towards Italy were tripped automatically and the country was isolated from the European network within seconds.
“About two minutes and 30 seconds after the disconnection of the country, the blackout was an unavoidable fact,” stated the report.
Swiss Federal Energy Office had no immediate comment but said the report would form part of its own investigation.
The blackout left some 55 million people in Italy without power – some for up to 18 hours.
In the wake of the September 28 power cut, the French and Italian authorities blamed Switzerland for the blackout.
The Swiss, French and Italian authorities demanded an investigation by the UCTE to find out what went wrong.
Commenting on the general power situation in Europe, the UCTE said that the continent's ageing power grid was not designed to handle the increasingly high level of cross-border energy sales.
"We are using a system now for situations for which it was not built," said Frank Vandenberghe, the chairman of the investigation committee.
swissinfo with agencies
3:01 A tree hits and trips line on Lukmanier pass
Other lines become overloaded.
3:11 Etrans calls GRTN in Italy requesting measures to relieve overloads.
3:25 A second tree falls and another line trips, this time on the San Bernardino pass.
3:25 and twelve seconds later, Italy becomes isolated from the European network.
3:28 Roughly 95% of Italy is without power.
In compliance with the JTI standards