The failure to fortify flour with folic acid is like having a polio vaccine and not using it.

Women are ignoring expert advice to take folic acid supplements before pregnancy to protect their unborn children, a study has shown.

Researchers who questioned nearly 500,000 women attending antenatal clinics in England and the Isle of Man found that fewer than one in three took folic acid prior to getting pregnant.

This was despite strong evidence that most cases of spina bifida and other birth defects affecting the brain, spine or spinal cord can be prevented by boosting pre-pregnancy levels of the B-vitamin.

The study showed that the proportion of women who heed the guidelines when planning a family had actually fallen, from 35% in the years 1999 to 2001, to 31 per cent in 2011-2012.

Even among women with previous experience of a pregnancy involving a neural tube birth defect such as spina bifida, only just over half (51 per cent) took the supplements.

The findings prompted calls for the introduction of mandatory folic acid fortification of flour in the UK – a policy already adopted in more than 70 countries including the US and Australia.

Nicholas Wald, from Queen Mary, University of London, one of the study authors whose original work uncovered the protective effect of folic acid, said: “It’s a public health tragedy that in spite of the folic acid fortification initiative in many countries, the UK has not introduced mandatory folic acid fortification.

“The failure to fortify flour with folic acid is like having a polio vaccine and not using it.”

The research, published in the online journal Public Library of Science ONE, was conducted by a team from Queen Mary’s Wolfson Institute of Preventive Medicine between 1999 and 2012.

It showed that more women took folic acid once they discovered they were pregnant, the proportion rising from 45 per cent to 62 per cent between the periods looked at in the study.

But experts stress that to offer effective protection, the supplements need to be taken before pregnancy.

The study also showed strong ethnic variations, with only 17 per cent of Afro-Caribbean women, 20 per cent of South Asian women and 25 per cent of East Asian women taking folic acid, compared with 35 per cent of white Caucasian women.

A mere six per cent of teenagers under 20 attending the antenatal clinics had taken the supplements, while 40 per cent of older women aged 35 to 39 followed the guidelines.

Co-author Jonathan Bestwick, a lecturer in medical statistics at Queen Mary, said: “The current UK policy of recommending women take folic acid supplements has failed and has also led to health inequalities among ethnic minorities and younger women.

“The government should introduce mandatory fortification of flour with folic acid without delay.”

Colleague professor Joan Morris, also a statistician at Queen Mary, said: “Each year in Britain there are about 1,000 pregnancies affected by spina bifida or other birth defects of the brain, spine, or spinal cord.

“Most of these lead to a termination of pregnancy which is an agonising decision for couples who want a child.”

Spina bifida is a neural tube defect that occurs when the developing spinal column does not close properly, leaving nerves exposed.

In most cases surgery can be carried out to repair the defect after birth, but often nerves have already been damaged leading to paralysis, incontinence and loss of skin sensation.

Among the known risk factors for spina bifida, the most important is a lack of folic acid before and at the very start of pregnancy.

Jane Munro, from the Royal College of Midwives, said: “There is no doubt about the benefits of taking folic acid supplements for women who are planning to become pregnant. The RCM advises women to take supplements if they can. However, there is a need to ensure access to supplements for women who are unable to afford them, and to reach groups of women where taking these supplements is low.

“On the issue of adding folic acid to foodstuffs such as flour, we would stress the need for more discussion before such a step is taken, because there will be people for whom additives will be unacceptable.”

Clare Murphy, from the British Pregnancy Advisory Service, the UK’s largest abortion provider, said: “Many pregnancies are unplanned, and so often women do not know that they need to be taking folic acid supplements until it is too late.

“Some of the saddest cases we see in our clinics are those involving couples who are ending a much-wanted pregnancy, often after a diagnosis of a neural tube defect like spina bifida.

“Countries which have introduced mandatory fortification of flour with folic acid have seen a significant reduction in affected pregnancies, and we urge the government to act for the benefit of pregnant women in this country too.”