MADRID (AFP) - The new number two at the European Central Bank warned on Saturday that ideas for a levy on bank profits, pushed as a way to create a buffer against future crises, need much more work.

• European bank tax could raise €50 billion a year
• Sweden calls for US-style EU bank tax

Portugal's Vitor Constancio, who was voted into office last month by leaders of the 16 euro nations, told reporters that proposals to tax banks, which emerged amid anger over huge taxpayer bailouts, could not be set up in isolation.

Both Sweden and the European Commission have come up with plans due to be discussed informally by European Union finance ministers meeting in Madrid on Saturday.

Constancio said levy ideas could only be properly assessed after tests on separate proposals to regulate banks' capital liquidity, "because we need to assess the overall impact of everything that is now being studied regarding future regulation of the banking sector."

"It's a little bit premature," he underlined.

"I also think that if governments want to go ahead, that this be linked with deposit guarantee schemes and possibly used only for restructuring and resolution problems of the banking sector itself, not for any other purposes."

Mario Draghi, who represents the Bank of Italy at the ECB, also insisted that the levy's broader economic impact -- with wide variations in suggested levels -- had to be examined fully first.

He said banks were already losing "more and more" through write-downs on credit losses.

Europe's financial services overlord, France's Michel Barnier, warned on Friday that deals on banking regulation were needed quickly if anything stronger than "artificial" controls are to be put in place.

"I hear ministers and bankers telling me: go easy, if you ask for capital requirements that are too great, we won't have any money left with which to finance the economy.

"I remind them of the costs of the crisis," Barnier underlined, adding it was "unacceptable for taxpayers, because we hadn't drawn up new mechanisms, to be forced to pay for the mistakes and over-the-top actions of certain actors within the financial system."