Petrol-Is to which the TUPRAS workforce is affiliated, applied to the State Council for the sale to be annulled.
''Our union has opened a case against the TUPRAS tender specifications. The sale tender based on this illegal document is also against the law,'' it defended in a written statement.
The consortium led by Turkish conglomerate Koc Holding and including Royal Dutch Shell made the highest bid of $4.14 billion for the 51% stake in Monday's long-awaited state auction.
The sale has given fresh impetus to Turkey's IMF-backed privatization program and is the second-largest Turkish sell-off after the summer's $6.55 billion sale of a 55% holding in fixed-line phone company Turk Telekom.