The Polish banking supervisor has estimated that a forced conversion of franc loans to zloty at an artificially low rate could cost them as much as $13 billion.
A Brazil-like currency crisis could follow, causing the zloty to plummet, and throwing Poland's course towards the euro into doubt.
Other Polish banks have been offering low-interest mortgage loans denominated in euros and Swiss francs which looked cheap, but have proved to be unexpectedly expensive for borrowers as the zloty has slid by 15% against the euro since the start of last year.But by and large, the foreign banks are winning their bet.
In the absence of a currency peg, they were perfectly willing to make loans in zloty through their Polish subsidiaries.
First, you need to know that Poland's currency, the zloty, was never pegged to the euro.
The strong currency also provided flexibility to respond to a downturn.Sure enough, when the crisis hit and Western European lenders started pulling their money out of Poland, the zloty lost more than a third of its value against the euro.
That slowed growth then, but proved a blessing later.Another advantage was a floating exchange rate: the Polish zloty plunged during the downturn, helping exporters and the tourism industry.
Since the zloty was floated in 2000, its value has fluctuated a great deal.
Now they are queuing up to buy government debt, and the Polish zloty has soared in value (see chart).
For those earning forints or Polish zloty, the big swings in exchange rates in recent weeks have sent the size of both loans and repayments spiralling upwards.
Outside Slodowiec station stands a large billboard bearing the European Union's blue-and-gold flag which explains that more than a third of the 858m zloty (€251m) cost of the new station came from EU funds.Not counting farm subsidies, estimated at €14 billion between now and 2013, Poland's slice of gross EU payments will come to more than €80 billion, or €2,000 for every Pole.
