Banks in Portugal, Ireland, Greece and Spain account for over two-thirds of the euro area liquidity injections since the middle of 2008, according to new research from RBS Marketplace.
Of the total 332 billion euros ($408 billion) pumped into the banking system by the European Central Bank over the period, 225 billion euros went to banks in the four countries, RBS Marketplace analysts wrote in a research note.
"As at end-May, the Eurosystem had injected 332 billion euros since the middle of 2008, equivalent to about 4 percent of euro area GDP," RBS analysts wrote. "Unsurprisingly, in absolute terms bank borrowing from the ECB in Greece, Portugal and Spain is at a record level in each country."
Analysts fear that euro zone banks will suffer because of the debt crisis which has started in Greece and threatens other peripheral members of the single currency area.
Greek banks have tapped the ECB for liquidity that is "15 times larger than its share of banking sector assets or nine times larger than its gross domestic product weight in the euro area," according to the RBS analysis.
Banks in France, Germany, Italy and Austria have used the ECB liquidity support far less, the research shows.
In April the Bank for International Settlements released a list of the banking systems that are most exposed to the turmoil.
Portugal's dependency on Eurosystem loans has increased over the past month, while Ireland's has eased.
"The increase in the reliance to the ECB repo operations in May was most pronounced in Portugal, where ECB lending doubled to 36.8 billion euros. However, Irish bank reliance on ECB funding is nearly 40 billion euros lower than at its maximum a year ago, but remains elevated overall," the RBS analysis said.
The big question is what happens next and the team over at RBS do not see the ECB being able to exit this policy anytime soon.
"The increased stress in the banking sector in the periphery will result in the ECB having to step up its purchase program, including private sector securities," they wrote.
"Further purchases of covered bonds beyond the expiry of the existing purchase program (at the end of this month), either through the securities markets program or an extension of the covered bond purchase program, looks the most likely next policy step," they added.