The Institute of Food Research has also said that it may take 20 years before the effects of increased consumption by millions of people become known.
"Fortifying UK flour with folic acid would reduce the incidence of neural tube defects. However, with doses of half the amount being proposed for fortification in the UK, the liver becomes saturated and unmetabolised folic acid floats around the blood stream," the Daily Mail quoted Dr. Sian Astley of the institute as saying.
"This can cause problems for people being treated for leukaemia and arthritis, women being treated for ectopic pregnancies, men with a family history of bowel cancer, people with blocked arteries being treated with a stent and elderly people with poor vitamin B status," she added.
The warning comes as the Food Standards Agency has approved putting folic acid in bread flour in belief that it would cut the rate of birth defects such as spina bifida in the unborn baby, which can cause serious disability. The Government has yet to make a final decision in this regard.
According to the researchers, the human body may struggle to break down folic acid in even half the amounts proposed for supplementing foods, suggesting that even two slices of fortified bread a day everyday could lead to problems in people who are not deficient.
"This has important implications for the use of folic acid in fortification, because even at low doses it could lead to over-consumption, with its inherent risks," said Dr. Astley.
She said that a liver flooded with folic acid might end up releasing it undigested into the blood, and that excess levels of folic acid in the blood could lead to bowel and breast cancer, and accelerate brain decline in some elderly people.
Dr Astley said 13million Britons did not eat enough folate, which could equally put them at risk of health problems, and as many as 900 pregnancies a year are affected by birth defects for the same reason.
"It's a complicated and confusing issue. We want to encourage those who don't eat enough to increase their intake, but we don't want people eating too much in a sledgehammer approach to fortification. We have to work out the groups it might be helpful for," she said.