By Balazs Koranyi and John O'Donnell FRANKFURT (Reuters) - The European Central Bank could be forced by time constraints to delay a funding boost for Greek lenders, a key step towards their reopening, as it waits for Europe to agree a financial backstop that ensures Athens can repay its debts. The bank had been ready to increase Emergency Liquidity Assistance (ELA) after Greece approved the bailout deal. An emergency funding boost would have helped to restore confidence after Greece was nearly forced from the euro -- a debate that challenged ECB President Mario Draghi's pledge that the currency was irreversible.