Debt crisis: Mario Draghi pledges to do 'whatever it takes' to save euro

His speech at an investment conference in London boosted markets and forced down Spain and Italy's borrowing costs.

"Within our mandate, the ECB is ready to do whatever it takes to preserve the euro. And believe me, it will be enough," .

"To the extent that the size of the sovereign premia (borrowing costs) hamper the functioning of the monetary policy transmission channels, they come within our mandate."

When asked what probability he would assign to the euro zone having the same number of members it has today in two years, he added: "I don't venture into speculations about things like changes in the treaty. The treaty was meant to have the number of countries that we see today, so frankly I can't really estimate the probability of that."

"We think the euro is irreversible."

Investors were heartened by Mr Draghi's stance. The FTSE 100 in London finished up 1.36pc at 5,573.16, while the IBEX in Madrid closed up 6pc at 6,368.80. The FTSE MIB in Milan closed up 5.62pc at 13,210.04, while the CAC 40 in Paris ended the day up 4pc, at 3,207.12.

Spanish 10-year bond yields fell to 6.8pc and Italy's fell to 6pc.

Mervyn King, governor of the Bank of England, also spoke at the event.

"Global finance needs to return to its original mission of mobilising private sector capital to finance real investment opportunities," he said

"The global economy ... is rather far from vibrant and the troubles in the euro area and around the world are inevitably affecting the euro area and the United Kingdom."

"I am very struck that events of the last few weeks have... (turned many into) enthusiastic advocates of going even further than the Vickers proposals."