* World at critical stage in downturn
* Brown warns against complacency about recovery
(Updates after speech, adds quotes)
LONDON, May 20 (Reuters) - International efforts to fix the global economic crisis must be even faster and more co-ordinated than was discussed by G20 leaders at the London summit in April, British Prime Minister Gordon Brown said on Wednesday.
The world is at a critical stage of the downturn and failure to take the steps needed to restore growth would lead to more job losses and business failures, he added.
"We will now need a level of economic co-operation between nations if we are to solve these problems that is unprecedented, far more than I thought a year ago, more than even I thought in April," Brown said in a speech to the Confederation of British Industry, a business lobby group.
"(It) has now to be achieved far more quickly than ever I could have thought a few months ago."
He told business leaders there can be no complacency about an economic recovery given persistent difficulties facing the global economy.
"We will not and must not relax our efforts to move this economy through the downturn, back to a period of growth," he said.
Brown's address follows a report from the International Monetary Fund which highlighted an easing in Britain's recession but also said high household debt and a lending squeeze could dampen recovery.
Finance minister Alistair Darling stood by his estimates for a recovery towards the end of 2009 in an interview with The Times newspaper on Wednesday, although forecasts from the Bank of England last week suggested the recovery could come later.
Despite the current challenges, Brown said the world economy could double in size in the next two decades, buoyed by millions of new consumers in China and Asia. To achieve that, the world economy needs better to be open, flexible and better regulated, Brown said.
"There is a huge prize for all of us as the global economy returns to growth," he told the CBI. "But it will not be sustainable growth if we do not deal with the problems that exist within this global economy." (Reporting by Matt Falloon and Peter Griffiths; Editing by Matthew Jones)