"Where it was not possible to use a qualified majority, it was used and we oppose that, as well as the fact that the right of a sovereign EU member to use the veto right was ignored," said Fico.
He added that the Slovak Government's position is that this represents a clear violation of all the principles on which the EU is founded.
Justice Minister Boris Suško said that the lawsuit will be filed next week.
Hungary, whose outgoing Prime Minister Viktor Orban said the EU had “shot itself in the foot” by imposing sanctions on Russia in response to the conflict in Ukraine, filed a similar lawsuit in February.
Fico also strongly criticized what he described as "suicidal sanctions" and called on the bloc to engage diplomatically with Moscow.
In January, the EU formally approved a plan to phase out Russian gas from the pipeline by 2027, overruling vetoes by Slovakia and Hungary.
