The union said the “murky events cast a shadow” over the battle for the next CalMac contract, being fought by Serco and David MacBrayne.
MacBrayne is the Scottish Government-owned parent company of both CalMac and the former NorthLink operator.
Streamline Shipping, another NorthLink bidder, is suing the Scottish Government in the Court of Session over its handling of the tender process.
Government agency Transport Scotland said the tender process in 2012 was “fair, balanced and properly conducted within strict procurement rules.”
Its spokesman said MacBrayne’s bid “simply did not meet the required quality score and consequently was failed.”
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