WARSAW, Sept 11 (Reuters) - Polish courts may check the validity of mortgage loans indexed to the Warsaw Interbank Offered Rate (WIBOR), but their assessment cannot refer to the WIBOR rate itself or the method for determining it, an adviser to the top EU court said.

Polish banks, still dealing with the consequences of turmoil related to foreign currency mortgage loans, now face further challenges as customers question whether some elements of loans indexed to WIBOR could have been abusive.

Some clients argue the way WIBOR was set - based on banks' declarations, not actual transactions - resulted in higher mortgage payments for them, and asked courts to rule on the issue.

The EU's Court of Justice was asked whether a court could question loans referenced to the WIBOR rate and if so, whether such a contract could continue to be in force with the WIBOR clause removed, resulting in the client paying only the bank margin.

"The fact that WIBOR is a critical benchmark ... does not in itself exclude the ... judicial review of a contract term that refers to WIBOR," court advocate general Laila Medina said in a statement.

"However, this assessment cannot refer to the WIBOR rate itself or the method for determining it."

The EU tribunal follows the opinion of its advocate general in a majority of cases, but its ruling on foreign currency loans differed from the opinion. A final ruling is expected in a few months.

The Polish government had been seeking to replace the WIBOR interbank rate on which most mortgages are based by a more transparent instrument.

Banks' stock on the Warsaw bourse did not react to the publication of the opinion, and the Polish Banking Association welcomed the court's interpretation, telling PAP news agency it confirmed the correctness of setting WIBOR and fulfilling information obligations by banks.

(Reporting by Pawel Florkiewicz and Anna Wlodarczak-Semczuk; Editing by Andrew Cawthorne)

((warsaw.newsroom@reuters.com))