The European Central Bank is facing calls to do more to tackle the eurozone debt crisis as speculation grows that Spain will need financial support and Greece might exit the euro. But the financial industry is starting to question whether the ECB has the strength of balance sheet to provide further help even if it wanted to.

Concern is increasing in the wake of the ECB's long-term refinancing operation in December and February, which allowed banks to borrow €1 trillion for three years on cheap terms. A year ago, the ECB's balance sheet was €1.9 trillion, according to Bloomberg - it is now close to €3 trillion.