A senior source in New York said Nestle had raised the issue of a "technical conflict" between Heinz and Maggi, the Nestle-owned soups and sauces brand.
McCann holds the Maggi business in markets including Scandinavia, South America and South-East Asia. The agency's only Nestle business in the UK is the Nescafe account.
McCann will continue to work on the Heinz account for the next three months, during which time Heinz will consider its options. McCann is understood to have produced enough work for a further 12 months of campaigns.
A source close to the business said Heinz was unlikely to call a review, leaving M&C Saatchi poised to scoop the Heinz business. Last November, it split the account with McCann, taking Heinz's Amoy account.