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ECB bank tests may cause ‘surprises and deleveraging’

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Euro zone bank shares tumbled Wednesday, after the European Central Bank (ECB) revealed tough new stress tests for the region's financial institutions.

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Mizuho International Chief Economist Ricccardo Barbieri attributed the fall to lack of detail on how the ECB will assess the euro zone's leading banks, coupled with fears the capital limits insisted on in the tests were more stringent than those currently posed by national regulators.

One hundred and twenty-eight banks will undergo an assessment of their risky assets, the quality of their balance sheets, and the amount of capital they hold. This is designed to test their ability to withstand economic difficulties or "stress".

"Now the ECB has to introduce a harmonized model, which will be potentially very complex. Therefore there could be surprises and deleveraging because of it," Barbieri said, following Draghi's interview with CNBC.

Draghi: Europe needs banking transparency

(Read more: Draghi: New bank stress tests 'just the beginning')

"Over time we will have to acquire more elements to assess what the ECB is really up to… the ECB will carry out checks on liquidity, capital, positions, overall leverage of banks… they are not telling us more for the moment about how they are going to do it, what will be the parameters. That introduces a bit of uncertainty and this is perhaps why today banks sold off," Barbieri added.

James Howat, European economist at independent research firm Capital Economics, agreed that market confidence was hit by a lack of detail issued by the ECB. He added that the stress tests appeared to be tougher than anticipated, particularly with regards to the 8 percent core Tier 1 capital requirement.

"It is difficult gauging the markets' reaction. If they (the ECB) are too severe it hits confidence, but it is also disappointing if they are not strict enough," he told CNBC.

The Euro Stoxx banks index traded 2.1 percent lower after the ECB's announcement, underperforming the broader Stoxx 600 index, which traded 0.70 percent down on the day.

Italian and Spanish banks were among the worst hit, with Mediobanca, Banco Popolare, Banca Populare di Milano and Bankia trading between 4.1 percent and 5.7 percent lower.

View more from Mario Draghi's CNBC interview here

—By CNBC's Katy Barnato. Follow us on Twitter: @CNBCWorld