The news was announced on Monday by the Frank Association, formed by Swiss franc borrowers after their loans became much more expensive when the Swiss central bank scrapped the cap on the franc's exchange rate in 2015.

In the ruling, the court said that the bank should have known that the currency would appreciate relative to the euro after the conclusion of the loan contract, the association said.

What is more, the bank should have known that there was a great possibility that major changes could take place in the economy or in markets in the long run that could reflect in major changes in currency rates or the reference interest rate.

The bank should have explained this possibility to the borrower, said the court, which has accepted the borrower's claim that the possibility of an appreciation of the Swiss franc relative to the euro and its consequences had never been presented.

The borrower thus never possessed information on whose basis he would potentially make a different decision, which is why the contract has been declared null and void, the court said.

The Slovenian branch of Unicredit bank has already announced an appeal, arguing that the ruling is based on "incomplete findings of the actual situation and deficient and, consequently, incorrect legal conclusions".

The bank told the STA that this was only one case on which the court of first instance ruled, so it expected a decision of a "higher court that would follow the convincing argumentation of other courts and the case law of the EU Court on the validity of loan contracts in foreign currencies".

Meanwhile, the Frank Association expects the ruling to encourage a bigger number of its members to file suits. It also pointed out that a special law should be passed in parliament as soon as possible.

A bill on the conversion of Swiss franc loans into euro loans has been announced by Boštjan M. Zupančič, a former judge at the European Court of Human Rights (ECHR) and at the Slovenian Constitutional Court. He expects it to be filed to parliament in the autumn.

Zupančič counts on media and political support and believes that the bill should be fast tracked through parliament.

According to the newspaper Delo, the bill was co-authored by another two former constitutional judges - Franc Grad and Ciril Ribičič.

Ribičič has told the paper that the bill relates to all loans in Swiss francs taken out between 2000 and 2010, with conversion applying to both repaid and open loans. Conversion would also apply to loans transferred by banks as non-performing assets to other legal entities.

According to the Frank Association, around 20,000 such loan deals were signed in Slovenia, a majority of them between 2005 and 2008. Most of them were 20-year housing loans.