The benefits of breastfeeding for mother and baby start from day one, say campaigners.
Breastfeeding has many health benefits for mother and child
Yet more than half of 500 women did not realise this, the National Childbirth Trust found.
The findings come as the NCT boosts efforts to encourage more breastfeeding.
A government poll of 427 men found 79% would want their baby to be breastfed, contrary to popular belief that they might not.
Breast or bottle?
The UK has one of the lowest breastfeeding rates in Europe - almost a third of women in England and Wales never try to breastfeed, compared with just 2% in Sweden.
In 2003, the government set a target to increase the number of women starting to breastfeed by 2% a year.
But experts acknowledge that breastfeeding is not suitable or the best option for all women and that individual choice is important.
The NCT surveyed women about their breastfeeding knowledge.
Nine out of 10 did not know that breastfeeding for just one month has a lasting impact on health during the first 14 years of a baby's life.
Although formula milk does not negate all the protection offered by breastfeeding, only a third knew that feeding babies under six months on both breast and bottle milk still provides them with a certain amount of protection from infections.
Almost half of respondents did not know that breastfeeding also reduces the risk of osteoporosis and cancer of the ovary for the mother.
Belinda Phipps of the NCT said: "Because the benefits start right from the first feed, any amount of breastfeeding is a reason to feel proud.
"This is especially important for the nine out of 10 women who stop breastfeeding before they want to, generally because of a lack of good information or support from those around them.
"It's encouraging that the majority of men are supportive of breastfeeding.
"However, with over a fifth of men not showing support, there is still more to be done."
Christine Carson, the Department of Health's National Infant Feeding Advisor, said: "We encourage every new mum to try breastfeeding.
"Sometimes it may not be easy, but with the right advice, support and encouragement, particularly from those closest to them, we hope to see a marked increased in the amount of mothers and their babies experiencing the many benefits that breastfeeding brings."