The boost comes from businesses securing contract wins, additional sales and new foreign investment in the last year, said the Government and London Mayor Boris Johnson.
The report said strong progress had been made against all the legacy commitments including an increase in volunteering, halting the steady decline seen since 2005, 1.4 million more people playing sport at least once a week than in 2005 when the bid was won and the legacy of all the permanent venues on the Olympic Park secured within a year of the Games.
Independent research projects that the total benefit to the UK from hosting London 2012 could reach up to £41 billion by 2020, said the report.
Prime Minister David Cameron said: "This £9.9 billion boost to the UK economy is a reminder to the world that, if you want the best, if you want professionalism, if you want jobs done on time and on budget then you should think British.
"With companies across the country we are harnessing the Olympic momentum and delivering the lasting business legacy of the Games that will help make Britain a winner in the global race."