Rob Reynolds, Al Jazeera's senior Washington correspondent, said Obama spoke to Brown on the flight from Washington, still trying at the last minute to co-ordinate a joint strategy for the meeting that begins on Thursday.
Explaining that some people in Europe were calling for Obama to start the meeting by apologising for the excesses of Wall Street and US financial institutions, our correspondent said Obama could be challenged on the central role the US played in the global financial meltdown.
Obama is expected to propose an array of proposals, including increased oversight for hedge funds and greater powers to deal with troubled financial firms deemed "too big to fail".
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White House aides have sought to manage expectations before the summit, telling reporters Washington was not necessarily asking countries to spend more right away.
After London, Obama will head to Nato's 60th anniversary summit on the France-Germany border.
Obama will use the meeting in Strasbourg to further explain his strategy for Afghanistan and urge Nato allies to commit greater resources to the fight against the Taliban and al-Qaeda.
He said last week that Washington will send an extra 17,000 soldiers and 4,000 advisers to train Afghan forces, plus hundreds more civilians to work on development issues.
PJ Crowley, an official in the administration of Bill Clinton, the former US president, said Obama may face resistance at the Nato gathering.
He said the improvement in tone "doesn't necessarily solve the problem that Afghanistan is a very complex situation" and persuading European allies to devote more resources to the war there at a time of economic crisis will be tough.