Several of the world's leading food companies are breaching an industry code limiting the promotion of breast milk substitutes to mothers with babies under six months old, according to a new report by a leading aid charity.

A survey for Save the Children found that care professionals in Pakistan were being targeted by breast milk substitute companies. The charity said it had also found evidence of "questionable marketing practices" in China.

Research firm Gallup interviewed 2,400 mothers and 1,200 health workers across Pakistan and found that a third of all available information relating to infant feeding was sponsored by commercial companies, in violation of the Interagency Group on Breastfeeding Monitoring code. The code was agreed in 1981 following widespread concern about the promotion of baby milk formula in the developing world.

Gallup reported: "Many items were giving misleading information and were promoting bottle-feeding in some ways. The findings suggest that the code is being violated in significant ways by companies in Pakistan, which try to influence mothers through interpersonal communication, advertisements and endorsement by health professionals."

Save The Children has amassed a powerful coalition of celebrities to back its breastfeeding campaign. Actress Isla Fisher, musician and model Myleene Klass and newsreader Natasha Kaplinsky have all helped publicise the report. Last week Victoria Beckham pledged support in a tweet to her 5.4 million Twitter followers.

Gallup found almost a fifth of health professionals in Pakistan had received a free gift from a breast milk substitute manufacturer. A further 10% reported being given free samples not just for the substitute but for follow-on formula milk, baby food and bottles and teats. Most of the gifts were stationery such as pens and prescription pads, sometimes branded, items which help health professionals reduce their own costs. There are also anecdotal claims that some midwives were given trips to Mecca, though it is not clear which firms are involved.

Two subsidiaries of Swiss giant Nestlé and French producer Danone, both named in the report, are the subject of an online petition calling on them to "stop any conduct that undermines breastfeeding". The petition has had almost 8,000 signatures. Save The Children said it would be raising the issue at the firms' annual meetings in the coming weeks.

Food companies see Asia as a major market for breast milk substitute, an industry already worth £16bn worldwide, and set to grow by 31% in the next three years. In east Asia and the Pacific, the number of breastfeeding mothers has fallen from 45% in 2006 to 29% in 2012. This is due in part, says Save the Children, to inappropriate marketing practices by some breast milk substitute companies.

The charity also blames the decline on traditional practices that eschew the use of breast milk, and a severe shortage of midwives and health workers in the developing world.

"Any marketing practices that break the international code could, we believe, undermine breastfeeding and therefore risk children's health," said Justin Forsyth, the chief executive of Save the Children. "We must act to ensure that these companies always stick to the code. In addition we are asking all manufacturers to change the prominence of the labelling on their products telling people that breastfeeding is the most effective way of protecting the health of their child. We would like to see this cover a third of the packaging."

The decline in breastfeeding is a major concern for aid agencies. They say that the use of breast milk substitutes brings heightened risks of contamination from unsterilised water. In a new report, Superfood for Babies, the charity claims that if babies receive colostrum, the mother's first milk, within an hour of birth, it kickstarts infants' immune systems, making them three times more likely to survive.

It also claims that, if the mother continues breastfeeding for six months, a child growing up in the developing world is up to 15 times less likely to die from killer diseases such as pneumonia and diarrhoea.

A spokesman for Danone said: "Infant formula is the only safe, legal alternative to breastfeeding and we believe an increase in the size of the warning label is counterproductive, in that it would send mixed messages to parents and potentially confuse them about which milks can be used safely for babies. Our products are safe and popular because they meet a real need for mums who choose to bottle feed. All of our infant formulas carry warning labels and meet strict legislative rules, and our customer communications reinforce the benefits of breastfeeding. Our priority is to standby mums and support them in their choices."

Nestlé said: "While we agree there is still work to be done to improve infant feeding practices and promote breastfeeding around the world, we believe Nestlé already has in place the main elements recommended in the report for improving breast milk substitute industry practices."