The world's leading central banks are facing the risk that their massive efforts to revive economic growth could be dragged down again, with some officials arguing for bold new ideas to counter the threat of slow growth for years to come.

A day after the U.S. Federal Reserve kept interest rates at zero, citing risks in the global economy, the Bank of England's chief economist said central banks had to accept that interest rates might get stuck at rock bottom.

In Japan, where interest rates have been at zero for more than 20 years, policymakers are already tossing around ideas for overhauling the Bank of Japan's huge monetary stimulus programme as they worry that it will be unsustainable in the future, according to sources familiar with its thinking.

Separately a top European Central Bank official said the ECB's bond-buying programme might need to be rethought if low inflation becomes entrenched. But he added monetary policy would not restore economic growth over the long term.

More than eight years after the onset of the financial crisis, the economies of the United States and Britain are growing at a healthier pace, in contrast to those of Japan and in many euro zone countries.

But the risk of a sharp slowdown in China and other emerging economies has prevented the Fed from starting to raise interest rates and is being watched closely by the Bank of England.

Investors mostly think that the Fed's delay will be short-lived and that it could begin raising rates before the end of the year, followed a few months later by Britain's central bank.

But the BoE's chief economist, Andy Haldane, who has long been gloomy about the chances of a sustainable recovery, said the world might in fact be sinking into a new phase of the financial crisis - this time caused by emerging markets.

In a speech on Friday that summed up the dilemma for many central banks, he said policymakers had a lot less room for manoeuvre than in the past.

Haldane pointed out that the Bank of England's next move might be to cut its rates below their record low 0.5 percent rather than proceed with a hike as widely expected. But even when interest rates do start to rise, he warned they could be pulled back again to near zero by future problems.

"If global real interest rates are persistently lower, central banks may then need to think imaginatively about how to deal on a more durable basis with the technological constraint imposed by the zero lower bound on interest rates," he said.

"That may require a rethink, a fairly fundamental one, of a number of current central bank practices."

RADICAL IDEAS

Haldane said possible solutions included giving central banks higher inflation targets or making their emergency bond-buying programmes a permanent part of their armoury, although both options had their downsides.

He raised a third and even more radical possibility: governments issuing electronic rather than paper money, making it feasible for central banks to apply negative interest rates and break the current floor of borrowing costs at zero.

One of the Fed's policymakers even called for the U.S. central bank to set a negative interest rate at Thursday's meeting. But for now, most investors are betting on a less dramatic path for the world's central banks.

Anna Stupnytska, global economist at Fidelity Worldwide Investment, one of the world's biggest fund managers, said the Fed and the BoE remain on course to carry out a few interest rate hikes while the BoJ and the ECB carried on buying bonds.

She hopes the combination of technological advances, more spending by governments on infrastructure and reforms to boost competitiveness will also eventually help move the world economy out of its low-growth, low-inflation rut.