Three family members took out a franc loan in 2008. Due to the change of the exchange rate following the decision of the Swiss central bank to end the franc's peg to the euro at the beginning of this year, the loan principal surged.

The plaintiffs claim the bank should have warned them about the risks.

"The contracts included provisions that the borrowers are aware of exchange rate risks...but in my opinion and according to case law, this is insufficient," said the plaintiffs' lawyer Robert Preininger.

He said the loan was a kind of bet, with the borrowers betting that the exchange rate would remain stable, and the bank counting on profiting on the exchange rate difference.

The key question therefore is what the banks knew: if they knew that the rate could change significantly, "they did not equally share this risk with the borrower," he said.

The bank's attorney did not wish to comment on the case, while the bank had previously said that the risks were evident from the bank's promotional materials.

The next hearing in the case is scheduled for 14 January.

The Franc Association, an informal advocacy group of franc borrowers, says almost 100 individuals have so far decided to sue their banks.

Franc loans had long been popular in Slovenia because interest rates were much lower than for euro loans. But when the franc rate spiked in January, the instalments of many borrowers surged.

Some countries, most notably Croatia and Hungary, took controversial measures to protect borrowers. The Slovenian government decided that action was not needed.

At the end of last year households had nearly EUR 800m in outstanding loans in francs.