A "statistical error" that resulted in an upper limit being set for daily folic acid intake may have contributed to hundreds of children being born with common birth defects, scientists say.
The London-based researchers looked at anencephaly - where a baby is born without parts of the brain and skull - and spina bifida - where the bones of the spine do not form properly around part of the baby's spinal cord.
They believe around 3,000 cases of babies being born with the two defects could have been prevented in the UK in the last 20 years if the British Government had approved a plan to make folic acid a compulsory ingredient in flour.
A new study, published in the journal Public Health Reviews, found the upper limit of 1mg a day to be based on "flawed analysis".
The researchers are now calling on the Government to approve fortification of flour with folic acid - which is a B vitamin - based on their findings.
White flour is already fortified with iron, calcium and other B vitamins (niacin and thiamin) in the UK.
The 1mg a day limit was adopted after the findings from the US Institute of Medicine (now known as the National Academy of Medicine or NAM) suggested those with vitamin B12 deficiency are at an increased risk of damage to the central and peripheral nervous system when consuming higher doses of folic acid.
This, according to study leader Professor Nicholas Wald, has been seen as a barrier to introducing fortification of folic acid in the UK.
But Prof. Wald and his team in London concluded there is no need for the 1mg a day upper limit as high doses of folic acid do not lead to neurological damage.
The researchers said this "flawed" NAM analysis from 1998 was a result of "misinterpreted data from 23 studies that they considered".
Speaking at a press briefing, Prof. Wald, from the Wolfson Institute of Preventive Medicine at Queen Mary University of London, said: "The Government has been doing due diligence - insisting on eliminating every shred of evidence of harm.
"It is the advisory board that has failed here - it has failed because of the most extraordinarily simple statistical error."
It is estimated that one in every 500-1,000 pregnancies is affected by anencephaly and spina bifida, which are collectively referred to as neural tube defects (NTDs).
Previous studies, which included a randomised trial in 1991, have shown that increasing folic acid intake immediately before and early in pregnancy prevented NTDs by at least 80%.
The US introduced mandatory folic acid fortification of cereals in 1998, which according to the researchers, has been found to reduce the prevalence of NTDs without any evidence of harm.
The number of NTDs has decreased by up to a half in the 80 other countries that have introduced fortification, they added.
Study co-author Professor Joan Morris, also from the Wolfson Institute, said: "From 1998, when the US introduced mandatory folic acid fortification, to 2017, an estimated 3,000 neural tube defects could have been prevented if the UK had adopted the same level of fortification as in the US.
"It's a completely avoidable tragedy."
Women looking to become pregnant are advised by the UK's Department of Health to take 0.4mg of folic acid supplement on a daily basis - from before pregnancy to until 12 weeks of being pregnant - to prevent the risk of NTDs.
Prof. Wald said: "Unfortunately, most women do not follow this advice, either because the pregnancies are unplanned or they don't know about folic acid.
"Only a third of women in Britain take folic acid while trying to get pregnant."
But the researchers add that even with fortification, pregnant women should still be advised to take folic acid supplements as a safety net.