PRWeek understands Blue Rubicon, 3 Monkeys, Synergy, Fast Track, Threepipe and Clifford French have been selected for the work.
The agencies have been lined up to help boost public interest in mid-sized sports amid widespread scrutiny of the Olympics' long-term sporting legacy.
Sport England's new PR focus follows the organisation being awarded £1bn in 2012 by the National Lottery and the Exchequer for a five-year campaign on youth and community engagement.
A letter seen by PRWeek suggests the selected agencies are now subject to a 'standstill period' until 22 July, to allow for agencies that failed to make the list to appeal if they wish.
One agency figure with an insight into the brief said that a key focus would be on 'working with different governing bodies to come up with campaigns to drive sporting participation'.
Last month, a Sport England survey found that the number of adults playing sport had dropped since the Games, with 280,000 fewer people aged over 26 playing sport in the past year.
However, the source said that though the Olympic legacy 'was always going to be part of it, not everything will be explicitly capitalising on the Games.
'What will be in focus is how different sports will be able to collaborate on different campaigns targeting similar audiences,' the source added.
PRWeek broke news of the pitch in March (prweek.com/uk, 13 March).
At the time, Sport England's director of business and partnerships Tanya Joseph stated that 'football, cricket and rugby' would not be a part of the remit.
The £1bn funding will cover a £450m investment in the national governing bodies of sport, facilities funding, local investment and extra-curricular sports activity around schools.
Sport England declined to comment.