The Flora 1 was boarded on Friday after officials alleged there were issues with its registration status and that the vessel was on an EU sanctions list.
But the coast guard said late on Saturday that Cameroon had confirmed the vessel's registration and no environmental offence could be proved.
"There are no further grounds for continuing the investigation," it added.
READ ALSO: Swedish coast guard boards suspected false-flagged tanker
European countries have ramped up action against a so-called 'shadow fleet' of vessels that transport oil from Russia in an effort to get around EU and other international sanctions.
According to ship tracking site marinetraffic.com, Flora 1 departed the Russian oil port of Primorsk on March 31st, heading for Santos in Brazil and flying the flag of Sierra Leone.
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