The study follows a World Wide Fund for Nature (WWF) report, which found more than 350 chemicals, from substances such as perfume, suntan lotion and pesticides, in breast milk samples.

The BBC's Margaret Gilmore: "These are chemicals which didn't exist 50 years ago"

Although many of these are not dangerous, it said British infants may be taking in 42 times the safe level of dioxins.

However, government experts, the WWF and child health specialists all said that the benefits of breastfeeding were far too great to stop.

And the WWF said that cow's milk, which is used in infant milk formulas, could be equally contaminated.

These poisonous chemicals can build up in the body over time and cause cancer, immune system damage and hormonal imbalances.

The WWF study in turn follows an independent report into the findings of a government investigation carried out three years ago into dioxins in breast milk in Britain.

The report found "significant" but not dangerous levels of dioxins.

Bottle-feeding with cow's milk or formula may not avoid the problem

A spokeswoman for the Ministry of Agriculture, Fisheries and Food (Maff) said: "In terms of the safe limits over a lifetime it was not a cause for concern.

"The total amount that we are talking about fits quite comfortably into the total amount that somebody is supposed to get, especially given that environmental exposure to this sort of thing has been falling over recent years.

"But what we felt was that this was an issue that warranted further study."

WWF's Elisabeth Salter: "Breastfeeding is still definitely best"

About 1,000 mothers are to be recruited from the Yorkshire area to take part in research led by Leeds University, paid for by the government.

The Leeds survey will attempt to collect a representative sample of breast milk to form an "archive" of milk samples reflecting social groups, diets, and number and position of children in families.

The government was keen on Sunday to stress that mothers should still breastfeed.

The Maff spokesman said: "The potential risk as a result of residual contaminants is far, far outweighed by the clear and proven nutritional, health and other benefits of breastfeeding."

That advice was repeated by Professor David Baum, President of the Royal College of Paediatrics and Child Health.

He told the BBC: "There is no doubt in my mind that mothers should continue breastfeeding.

"The mother and baby are a nutritional unit from conception onwards. There are no reports of any health disadvantages."

He did call for more research into the issue: "It's a very proper concern that there should be contamination in our food cycle.

"But I see nothing in this report which should lead any mother or father to waver."