Opening a new phase in a bitter worldwide controversy, the three American manufacturers of infant formula pubicly launched a joint campaign yesterday to defeat a proposed international marketing code for breast-milk substitutes.

Executives of Bristol-Myers Co., Abbott Laboratories and American Home Products Corp. began a two-week series of visits to officials of all affected federal agencies and to key legislators to urge that the United States oppose adoption of the draft code, which makes detailed recommendations intended to promote child health and nutrition.

The draft, approved unanimously by the 30-member executive board of the World Health Organization (WHO) last Jan. 28, comes to a vote in May at the 34th World Health Assembly in Geneva. The assembly's more than 150 other member governments are being pressed by foreign formula-makers to reject the draft, but a multi-nation network of consumer, religious, nonprofit and health groups is urging them to support it.

In a three-page position paper, the companies denounced the draft as "a set of highly specific and restrictive rules that would virtually eliminate legitimate competition and promotion of infant formula products even to the medical community."

Up to now, the controversy has centered on promotional practices alleged to deter breast-feeding in Third World nations, where water for mixing substitutes is often polluted and the bottles dirty. Bristol-Myers says, however, that the proposed code also would affect "legitimate" and "proper" promotion of formula to nurses and physicians in the United States.

The three U.S. firms said they had "worked diligently" to help WHO and UNICEF, the United Nations children's fund, develop ethical formula marketing. But they attacked the draft code as "a serious distortion of the original intent" of the two U.N. bodies.

Disagreeing, Douglas A. Johnson, speaking for a coalition of groups favoring the WHO/UNICEF code, said the code would be "only a recommendation." He accused the companies of a "reprehensible" turning away, in a changed political climate, from a code they themselves had encouraged.

Mead Johnson and Abbott's Ross Laboratories sell most of their output in the United States, which accounts for about one-quarter of an estimated $2 billion in annual formula sales. American Home's Wyeth Laboratories sells mainly overseas, where the dominant seller is Nestle, the giant Swiss foods conglomerate, and where a dozen Swiss, Dutch, Japanese and British firms also compete. Nestle does not sell formula here.

The draft grew out of a 1979 meeting sponsored by WHO and the United Nations children's fund.

One of their chief recommendations was that the marketing of infant formula should not discourage breastfeeding. No one disputes that breastfeeding, when a mother can offer it, provides near-perfect initial nutrition.