By Imogen Foulkes BBC correspondent in Bern

The move follows criticism from the Italian authorities that the Swiss did not inform them quickly enough about damage to electricity lines.

The blackouts left thousands stranded

It also comes in response to news that not one, but two major power lines from Switzerland to Italy went down in the run up to the blackout.

The formal investigation will reconstruct the events leading up to the power blackout.

Log books will be reopened, measurements re-examined and Swiss power workers on duty at the time will be questioned.

The Swiss authorities are tired of the war of words with Italy, but they are also uncomfortable at the latest evidence which proves that not one, as originally stated, but two major power lines from Switzerland to Italy were disrupted in the 20 minutes before the black out.

They were both apparently damaged by trees blowing against them, causing electrical arcs which knocked out the power supply.

Conflicting claims

The Italians say the Swiss did not inform them quickly enough of the damage.

The Swiss say they did issue a warning, but the Italians did not respond fast enough.

The purpose of the investigation is to find out exactly who said and did what and when.

Results of the inquiry are due by the end of this month.