Greek banks are running short on the collateral they need tostay alive, a crisis that could help force Prime Minister AlexisTsipras's hand after weeksof brinkmanship with creditors.

As deposits flee the financial system, lenders use collateralparked at the Greek central bank to tap more and more emergencyliquidity every week. In a worst-case scenario, that lifeline willbe maxed out within three weeks, pushing banks toward insolvency,some economists say.

“The point where collateral is exhausted is likely to be near,”JPMorgan Chase Bank analysts Malcolm Barr and David Mackie wrote ina note to clients May 15. “Pressures on central government cashflow, pressures on the banking system, and the political timetableare all converging on late May-early June.”