Europe's highest court will rule next week whether a bank in Poland broke the law by selling a Swiss franc mortgage.

The European Court of Justice (ECJ) is due to rule on 3 October whether it was legal for Raiffeisen Bank to grant foreign currency-denominated mortgages – a mortgage which is repayable in a currency other than your native one – in the Swiss franc currency to future Polish homeowners.

A sharp rise in the value of the franc has meant that many Polish borrowers have been left with huge debts.

The ECJ was reportedly asked by a Polish judge to see if these mortgages broke European law.

"The (ECJ) judgement will cause an avalanche of lawsuits,"  Janusz Szewczak, a lawmaker for Poland's ruling Law and Justice party, said.

Many borrowers have already turned to the courts, mostly disputing how banks set rates and how they alerted customers of risks. The number of cases lodged in Polish courts in the first half of 2019 rose by 39% to 2,021, justice ministry data shows.

If the judges rule in favour of borrowers, it could set a legal precedent that could cost Polish banks millions.

Almost 20 years ago, 700,000 Polish people took out mortgages denominated in Swiss francs – which had an interest rate far lower than those available in the Polish zloty currency. This meant that interest repayments were lower.

Foreign currency loans like his make up almost one third of all Polish mortgages, totalling 124 billion zloty ($31 billion).

Mortgage deals like this also became a problem in other European countries, including Hungary, Romania and Croatia, but Poland has been slower to resolve the issue.

Many Polish homeowners are now having to pay far bigger instalments than they expected, because of a sharp rise in the value of the Swiss franc.

Anna Wojtowicz-Bartlomiejczuk said she was left with huge debts when the franc's value jumped as she was paying off her foreign currency loan on a two-bedroom flat.

"It's like I have thrown the cash out the window."

The Swiss franc has risen 85% from 2.18 zloty to 4.04 since the collapse of Lehman Brothers in 2008.