According to work carried out by the Food Standards Agency (FSA) the fall in levels are in line with the results of recent surveys for these chemicals in other foods.
But while testing for dioxins appears to have cut contamination, yesterday's report on the increasing levels of polybrominated diphenyl ethers (PBDEs), a chemical commonly used in flame retardants, in fish oil products suggests that tests for new chemicals may be needed.
Dioxins and PCBs are long-lasting environmental pollutants formed as unwanted by-products of combustion processes, such as waste incineration, bonfires and cigarette smoke.
Concerns about the impact of dioxins and PCBs on people's health are particularly rooted in the potential risks to health that could come from long-term consumption of foods containing high levels of dioxins.
Under the FSA survey, samples of infant formula on sale in shops in the UK in 1998, 2001 and 2003 were analysed for levels of dioxins and PCBs. On the basis of the FSA research, infants fed on milk-based formulae would not exceed the Tolerable Daily Intake for dioxins and PCBs (2 picograms WHO-TEQ/kilogram of bodyweight/day).
The FSA reports that although there are no limits designated specifically for dioxins in infant formula all of the milk-based samples tested in 2003 were within existing European Union limits for dioxins in milk or milk-based products.
The same applies to soy-based foods for which there is no limit for dioxins specifically in these foods, but the FSA found that the concentration of dioxins in the soya-based formulae were also well below the limit that applies to milk-based products.
"However, as dioxins and PCBs build up in the body over many years, and it is long-term levels in the diet that are of concern rather than small variations over a few months, parents are not advised to switch to another brand or type of infant formula," said the FSA in a statement this week.
In general levels of dioxins and dioxin-like environmental pollutants PCBs in food have fallen by around 50 per cent in the UK over three years, added the FSA.