The economic outlook is cloudy from Canary Wharf 's financial towers
A leading economist has warned that the world could face five years of economic slowdown and a 10% decline in output.
DeAnne Julius, chairman of think tank Chatham House, said there was a 40% chance of such a lengthy slowdown.
Speaking at a G20 breakfast seminar organised by BBC News, she said an earlier recovery would depend on plans to restore banking lending.
Official forecasts have been much more optimistic, suggesting that the world economic recovery could begin as soon as next year.
The International Monetary Fund, for example, suggests that the world will grow by 1.5% to 2% in 2010 after declining by 0.5% to 1% this year.
DeAnne Julius' pessimistic outlook was shared by businessmen from G20 developing countries who took part in the seminar.
Igor Kalashnikov, who runs a double glazing company in Russia, said business was down two and a half times and there was a climate of pessimism among the media and policy makers.
And Nish Kotecha said that Indian start-up companies were finding it hard to get access to funding to expand their business.
Oktay Gokyildirim, who imports sweets into Turkey, said he has been badly affected by exchange rate fluctuations, but hoped that the G20 summit would give a boost to business and consumer confidence.
Other participants warned that politicians were building up too high expectations on the outcome of the G20 summit of world leaders, which takes place in London next week.
G20 LONDON SUMMIT
World leaders will meet next week in London to discuss measures to tackle the downturn. See
our in-depth guide
to the G20 summit.
The G20 countries are Argentina, Australia, Brazil, Canada, China, France, Germany, India, Indonesia, Italy, Japan, Mexico, Russia, Saudi Arabia, South Africa, South Korea, Turkey, the United Kingdom, the US and the EU.
Jim O'Neill, the chief economist of Goldman Sachs, said that it was unrealistic to expect countries to agree coordinated action to fight the downturn by increasing government spending.
Budget decisions would remain under control of national Parliaments, he said, and central banks, which were now mostly independent of governments, would also take their own decisions.
He was also sceptical about the G20's ability to move to an international system of financial regulation.
Without better national regulation, any attempt at international standards was doomed to failure, he reasoned, insisting that neither the US nor China would be prepared to have their banking regulations dictated by a new international body.
Countries like Brazil want a bigger role on the world stage
But Mr O'Neill, who coined the term Brics to describe the fast-growing economies of Brazil, Russia, India and China, said that the summit could be very important for changing the way the international economy was run.
He said he was very encouraged by the preliminary agreement at the G20 finance ministers meeting to restructure international financial institutions such as the International Monetary Fund (IMF) to give more power to countries such as China, and to increase the resources of the IMF to tackle the crisis.
The G20 was in danger of ignoring the needs of the world's poorest, said Vassi Naidoo, a partner in accountancy firm Deloitte who has worked widely in Africa.
Mr Brown is touring the world in search of a G20 deal
Mr Naidoo said poor countries have been badly affected by the collapse of remittances from people who have migrated to the rich countries .
These flows have represented a "lifeline" and the money needs to be replaced, he said.
He also described the global recession as a "catastrophe" for many African countries that are dependent on commodity export, citing the example of diamond exports from Botswana, which are down 60%.
He argued that the aid promised by the rich countries has to be maintained and increased to ensure that poverty goals were met, and promises to free up trade in agricultural goods had to be honoured.
Many experts and business leaders agreed that a key task of the G20 summit was to encourage free trade and avoid protectionism.
But there was a fear that nationalism was re-emerging despite the rhetoric.
Jim O'Neill spoke for many when he said that "words have to be matched by deeds" at the summit.