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.
Business Secretary Vince Cable said: "There is no doubt that the London 2012 Games proved a success story for British business. This has created a multibillion-pound business springboard to take British expertise to the world, and UKTI (UK Trade and Investment) is working hard to make the most of this significant opportunity. UKTI has embraced the challenge of delivering £11 billion of economic benefit from the Olympics. Reaching £9.9bn in the space of one year is an amazing achievement. I hope that companies of all sizes up and down the country follow suit and use the positive impact generated by the Games to reach out to new markets and grow their businesses."
Mr Johnson said: "Since last summer, the capital has seen a surge in overseas investment totalling billions from Croydon to Battersea to the Royal Albert Docks as well as seeing an extraordinary transformation of East London. This is delivering tens of thousands of jobs not only in London but spurring growth across the UK and helping the country sprint ahead in the global race for business."
As the first anniversary of the London Games approaches, International Olympic Committee president Jacques Rogge predicted there is "no doubt that the citizens of London and Great Britain will benefit from the Games for a long time to come". Mr Rogge said: "A year ago, the London 2012 Olympic Games wrote a thrilling chapter in the annals of Olympic legacy. London's commitment to delivering a strong Games legacy was clear, and plans for sustainable legacies were explicitly detailed in the city's initial bid to host the Games.
"These happy and glorious Games are now on their way to leaving a fantastic legacy that will benefit the population of London and beyond, whether it be the regeneration of a massive industrial wasteland in east London, the upgraded public infrastructures and transport, the employment and business opportunities, or the creation of the next generation of sporting champions by inspiring and enabling young people everywhere to become more involved in physical activity. The London 2012 Games have definitively served as a catalyst for development and improvements, both tangible and intangible, which would otherwise have taken decades to achieve. "