Greek banks own 'systemically important' banks in these south-east European countries and the worry is that if Athens were forced out the euro zone, its lenders would become bankrupt, with a domino effect on their subsidiaries.

If the various countries' governments then had to bail them out it would hurt national finances and longer-term growth prospects as banks reined in lending to try and stay afloat.

"We don't rule out the possibility of government support (for banks)," S&P said in a report. "If such support substantially weakens governments' fiscal and debt metrics, this could weigh negatively on our sovereign ratings."

Greece's Alpha Bank (>> Alpha Bank S.A.) has the biggest geographic spread across the region with arms in Romania, Bulgaria, Serbia, Albania and Macedonia. Eurobank Ergasias (>> Eurobank Ergasias SA) and Piraeus Bank (>> Piraeus Bank SA) are both in Romania and Bulgaria with the former also in Serbia.

S&P said the banks may raid subsidiaries for their best assets in a fight for survival and central bank efforts to ring fence them may be unable to stop runs on deposits.

S&P currently rates Bulgaria at BB+ with a stable outlook, Romania BBB- stable, Macedonia BB- stable, Albania B positive and Serbia BB- with a negative outlook.

It estimates that the market shares of the Greek banks' subsidiaries' range from about 15 percent of total financial system assets in Romania and Serbia to more than 20 percent in Bulgaria and Macedonia.

(Reporting by Marc Jones; Editing by Ruth Pitchford)

Stocks treated in this article : Piraeus Bank SA, Alpha Bank S.A., Eurobank Ergasias SA