A senior source in New York said Nestlé had raised the issue of a "technical conflict" between Heinz and Maggi, the Nestlé-owned soups and sauces brand.
McCann holds the Maggi business in markets including Scandinavia, South America and South East Asia. The agency's only Nestlé business in the UK is on 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.
Suzanne Douglas, the Heinz UK chief marketing director, said: "This is no reflection on McCann or the relationship with the Heinz team. It is also no reflection of the excellent strategic and creative work for Heinz Tomato Ketchup, Big Soup and Salad Cream that has been developed since McCann was appointed in November last year."