The CPI(M) said that the the over two decades-old case was sought to be used to "besmirch" the image of Vijayan and the party and added the court verdict has "effectively exposed these efforts".
"The judgement is a resounding vindication of the stand taken by the CPI (M) that the Lavalin case was a politically motivated case," the Politburo said in a statement, but refrained from naming any party.
Justice P Ubaid, who also upheld the discharge of two others in the Rs 374.50-crore graft case, rejected the CBI's plea that their full-fledged trial was necessary to prove the conspiracy.
The case relates to awarding of contract to Canadian company SNC Lavalin for renovation of three hydel projects -- Sengulam, Pallivasal and Panniyar in 1995 when Vijayan was power minister in the then E K Nayanar cabinet.