This is considered as the most serious (fault) in the industry," Bjoern Karlsson, chair of a safety body that advises the Swedish nuclear power inspectorate (SKI), told Swedish public radio SR.

The Forsmark 1 reactor was shut down on July 25 after a short-circuit caused a blackout. Two of four backup diesel generators failed to start automatically, revealing other faults in the power station's electrical system.

Karlsson was commenting on a technical report compiled by Forsmark staff published on Monday. According to him, information contained in the report shows the incident at the plant was the most serious in the history of Swedish nuclear power.

SKI was forced to halt operations at three other of the country's 10 reactors after the incident at Forsmark.

The blackout was classed by SKI as a level two incident on a scale from zero to seven.

The body on Thursday played down the significance of the incident, although the four power plants hit by the shut-down remain off-line.

"We were a long way from what could be classed as a serious incident," SKI spokesman Anders Joerle told AFP.

SKI's report on events at Forsmark is to be published later in the year, Joerle added.

Sweden has closed two of its original 12 nuclear reactors since 1999 as part of a plan to phase out nuclear power over the next 30 years, or when the reactors' lifespan expires.

Nuclear power accounts for nearly half of Sweden's electricity production.

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