PRWeek understands that Freuds will handle PR for the ticketing, volunteering and torch relay campaigns, which together comprise a majority of the 2012 PR budget.
Hill & Knowlton and Edelman, the other agencies that took part in the final round of the four-month long pitch process, also join the roster to handle specific projects.
Mandate Communications is also poised to take a spot on the roster, to continue its oversight of London 2012's public affairs, which it has handled since 2005.
A source involved in the review said that 2012 comms chiefs had a change of heart during the recent Vancouver Winter Olympics.
'They ran the process with the intention of coming to a single agency, then realised a roster was more sensible,' he said.
The pitch began in September 2009, with six agencies selected to tender for the business at the end of last year.
The original brief covered a wide range of activities, including PR for event, licensing, ticketing, mascot and education programmes.
Public affairs and political monitoring was also included.
The overall value of the business was estimated at approximately £2m over the next two-and-a-half years.
Freud pitched for the business in conjunction with sister Publicis Groupe agency MS&L Group, as well as Pitch PR.
Other agencies that took part in the first round of the pitch included Weber Shandwick and Bell Pottinger.
'We are in the process of appointing a number of roster agencies to work on a range of campaigns between now and 2012,' said a spokesman from the London Organising Committee of the Olympic Games and Paralympic Games (Locog).
'The procurement is ongoing.'