Both retailers said this week that they were in talks with the FPC about the possibility of linking with Tesco and Asda, the only other multiples to have pledged full support.
A Somerfield spokesman said: "Our produce team are in discussions with the FPC. We are very keen to support this."
The news was a fillip to the FPC, which this week announced the make-up of a board to lead the campaign - but also admitted it needed more cash. In a statement it said: "More work will be required to encourage many more companies within the fresh produce industry to pledge their support."
On the campaign board are Asda produce buying director Nick Burr and Tesco produce director Peter Durose.