European courts should rule on whether former Italian Prime Minister Silvio Berlusconi must sell most of his stake in asset manager Banca Mediolanum as ordered by Italy's central bank, a court adviser said on Wednesday.

In 2014 the Bank of Italy ordered Berlusconi's group Fininvest, which owns 30 percent of Mediolanum, to reduce its stake to below 10 per cent, as Berlusconi was no longer deemed fit to own a larger stake because of a tax fraud conviction.

An Italian court asked the Court of Justice of the European Union (ECJ) whether national courts could handle the case.

Advocate General Manuel Campos Sanchez-Bordona said in his opinion on Wednesday that, as the European Central Bank had the final decision on the matter, it was up to European courts to judge such a decision.

"The power to take the final decision is concentrated exclusively in the ECB, so jurisdiction to review judicially the exercise of that concentrated power must lie exclusively with the General Court and the Court of Justice," Campos Sanchez-Bordona wrote.

The Court usually follows the adviser's reasoning but is not required to do so.