The world cannot afford another financial crash like that of 2008, the Bank of England's head of financial stability has said during a visit to Dublin.
Andrew Haldane said that in the UK alone, the implicit guarantee to the banks amounted to £100bn.
"All this gives an incentive to the big banks to become bigger and more concentrated," Mr Haldane said. "This is the origin of the 'too big to fail' problem.
"Bankers had a 20-year party, but their market value relative to assets is back where it was 20 years ago."
Analysis suggested there were things regulators could have done to prevent such a huge crash, Mr Haldane added.
"They could have insisted on higher loan to value ratios as asset values rose," he said. "They could have made banks hold more capital as their loans expanded.
"If higher capital ratios had been in place in Ireland and the UK, it is possible that the credit boom would not have got out of control.
"With the permanent loss of output from the crash estimated as at least 100% of global GDP, the world could not afford another crash on this scale.
"The risk of another one will have to be combated with more and better regulation.
"Things like that are always said after a crash, and saying them wasn't much of a success this time."
Mr Haldane said he was optimistic that the new forms of capital, such as contingent convertible bonds, or Cocos, could change incentives for the banking system.
These are loans to banks which convert into equity if the bank has trouble meeting its debts.