The ruling lengthens the odds on a court victory for Berlusconi as EU judges have a track record of ruling in the ECB's favour.
The EU's Court of Justice said it alone had power to rule on the legality of a European Central Bank decision opposing Berlusconi's stake buy, which followed a procedure by the Bank of Italy.
"The EU Courts alone have jurisdiction to determine, as an incidental matter, whether the legality of the ECB’s decision...is affected by any defects of the preparatory acts adopted by the Banca d’Italia," the Court of Justice of the European Union said.
It was responding to an Italian administrative court, which had received a request from Berlusconi to suspend an order that his family holding company Fininvest cut its stake in Banca Mediolanum to below 10 percent.
Berlusconi also filed a parallel complaint to the EU's General Court, a branch of the Court of Justice that deals with cases against the bloc's institutions.
(Reporting By Francesco Canepa; Editing by Jon Boyle)
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