SINGAPORE, July 20 (Reuters) - Taiwan's Formosa Petrochemical Corp has slashed runs at its 540,000 barrels-per-day (bpd) Mailiao oil refinery to about 68%, down from 80%, following last week's shutdown of a secondary unit due to a fire, its spokesman said on Monday.

It was unclear when the affected 80,000-bpd residue desulphuriser will be able to resume operations as investigations are ongoing, said spokesman K.Y. Lin.

"Shipments of gasoline and diesel in August however would be affected as a result," he added, without elaborating on the total volumes affected. (Reporting by Seng Li Peng and Shu Zhang; Editing by Christian Schmollinger)