Lactalis, which owns President, issued a joint statement with Publicis confirming the two were ending their ten-year relationship by "mutual agreement".
The food company is now seeking an agency to handle its UK business. Last year, Lactalis spent about £500,000 in the UK.
Save the Children has hired Publicis to create a campaign to raise awareness of child poverty. The project is an integrated brief that will comprise press, posters, digital and direct marketing.
The charity already has relationships with the below-the-line agencies Proximity and Outrider.
A Save the Children spokeswoman said it would continue to work with those agencies on a project basis.
More than one million UK children live in severe poverty, according to Aimee Hansen, the UK regional communications and fundraising support manager at the charity.