MILAN, May 28 (Reuters) - Magistrates investigating Tuesday's incident at Saras SpA's <SRS.MI> refinery in Sardinia have sequestered the hydrocracking plant where three workers died, a spokeswoman for the Italian oil refiner said on Thursday.

Police have said the contract workers died on Tuesday after inhaling toxic fumes while cleaning a storage tank at Saras' mild hydrocracking number one plant at the Sarroch refinery, which was undergoing planned maintenance.

"The only thing that has been sequestered is the mild hydrocracking plant number one," the Saras spokeswoman said, adding investigators want to understand how the incident took place.

The move would have no economic impact since the plant was already halted for maintenance, she said. Saras is Italy's third biggest oil refiner by market share.

The spokeswoman said she could not give any details on any wider economic impact from a stoppage on Wednesday by Saras workers to mourn the dead nor from a planned strike on Thursday by contract workers.

Refinery production continued on Wednesday as the company managed the strike and worker coverage for the plant, she said.

Saras shares were down 1.9 percent at 2.0675 euros at 0757 GMT, about twice the downturn in the DJ Stoxx energy index <.SXEP>. (Writing by Nigel Tutt; Editing by Jon Loades-Carter)

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