World leaders are to try to inject new confidence into the ailing global economy, in a key test of Gordon Brown's leadership and diplomacy.
US president Barack Obama has warned that agreement at the G20 summit in London is essential if the world is to avoid mistakes that led to the Great Depression of the 1930s.
Hopes of a deal hit a stumbling block after French president Nicolas Sarkozy and German chancellor Angela Merkel warned they would not sign up unless there is tougher regulation of financial markets.
At a joint news conference, they said that it was a "red line" issue and that they would speak with "one and the same voice" in the talks at the Excel conference centre.
While there is a general belief that failure to reach an agreement would be unthinkable - sending financial markets into a tailspin - Mr Brown may have to endure some tense negotiating before he gets his deal.
Mr Sarkozy in particular has faced accusations of grandstanding for the benefit of a French domestic audience, a claim he angrily brushed aside insisting that his hardline stance was nothing to do with "ego or temper tantrums".
Nevertheless the French president was a pointedly late arrival at a working dinner for the leaders at No 10.
Downing Street insisted that the pre-summit negotiations were making "good progress" although the Prime Minister's spokesman acknowledged that there were "still a number of issues that need to be ironed out".
No 10 would not comment on a report by The Daily Telegraph that the final communique would include a key Franco-German demand - a worldwide cap on bankers' pay and bonuses.
