Worral Thompson said the decision to launch the body had been influenced by the 'powerlessness' of consumers in the face of supermarkets.
The body will look into competing with the supermarkets to drive prices down and actively promoting farmers' markets.
Positive Profile MD Henry Gewanter said the agency won the business via one of its consultants, Malcolm Murray, who had retained the agency while MD of fund managers Foster and Braithwaite.
Gewanter said the agency's work with similar campaigning organisations had been key to its winning the work: 'We've done PR for a number of big consumer champion groups, such as Energywatch. The aim is to position The Organic Partnership in the same way.
'We want to get members of the public signing up to the organisation, and then pass money on to the Soil Association to help them open regional offices to assist farmers in converting to organic methods,' Gewanter added.