Central banks tend to find transparency difficult when it comes to declaring the reasoning behind their monetary policy to investors and the European Central Bank (ECB) is a prime example, according to Citigroup's top global economist.
"Openness is not something that comes naturally to central banks in general, and the ECB is no exception," Willem Buiter, chief economist at Citigroup, told CNBC Thursday, when asked whether the German public should be made of aware the unstated aims of the ECB.
"One of the undeclared purposes of the (ECB'S) asset purchase program is to keep sovereign spreads in the periphery," the former Bank of England rate-setter said.
Buiter also suggested a key reason for the ECB to continue with its massive asset-buying program was to prevent sovereign debt spreads, such as those in Italy and Portugal, from spiking "significantly higher".
The ECB held the euro zone's benchmark interest rate at zero percent and left monetary policy unchanged at its April meeting. President Mario Draghi also confirmed the central bank planned to maintain its monthly pace of quantitative easing (QE) until at least the end of the year.
"We will face the challenge (of spreads spiking) of course when they do finally stop purchasing because then the hour of truth comes. Will these countries be able to develop growth in a sustained way, not as a cyclical recovery… or do we risk a blowout and risk debt restructuring that we had in Greece and Cyprus?" Buiter said.
Elsewhere, the Bank of England left interest rates unchanged at 0.25 percent, as expected, on Thursday. The U.K.'s central bank also said it may need to raise interest rates before the end of 2019 should Britain be able to successfully negotiate a smooth exit from the European Union in two years' time.
Ahead of the announcement, Buiter described it as a "non-event" and anticipated, "the Bank of England will just sit and have a cup of tea."