Martin Hellwig said stress tests carried out by the European Central Bank revealed the Deutsche Bank would be left in a precarious position in the event of another financial crisis.

While it would probably not go bust in a fresh downturn - he predicted the bank which is crucial to the German economy would face serious equity problems.

He said: "Putting it short: for a long and serious crisis there simply wouldn't be enough money."

The Berlin government has previously only bailed out the banks under extreme circumstances but Mr Hellwig, director of the Max Planck Institute for Research on Collective Goods, backed the idea of using taxpayers' money to fund public sector investment.

He said: "Turning banks into community property through public funds is not only possible but also necessary.

"If a bank is no longer able to help itself, the federal government should take on shares and exercise the related control functions."