H&K will promote the bid internationally with the ultimate aim of convincing the 125 members of the International Olympic Committee to vote for London ahead of rival cities.
London 2012 comms and PA director Mike Lee said H&K had emerged as London 2012's "preferred agency" but stressed that contracts had yet to be signed.
The IOC will reveal which of the nine applicant cities have made its shortlist on May 18, with the final decision due in July next year. Each city's PR work is limited by IOC rules.
London faces competition from cities such as Paris, which is being promoted by Weber Shandwick/Jon Tibbs Associates, plus Burson-Marsteller-backed Moscow.
H&K chairman David McLaren and sports marketing and sponsorship MD Alun James will lead an international agency effort, spanning corporate, consumer and PA teams.
GCI and QBO Bell Pottinger were eliminated from the pitch after its second stage, while Ketchum dropped out after initial discussions.
The final round of the pitch, which was coordinated in tandem with the COI, was overseen by bid chairman Barbara Cassani.
If you have an opinion on this or any other issue raised on Brand Republic, join the debate in the Forum here.