The warning comes as UK experts call on the Department of Health to add folic acid to flour to help reduce the incidence of serious birth defects including spina bifida.

But Professor Sander said that even if flour was fortified with folic acid, it would not constitute enough of the vitamin to reduce the risks for women with epilepsy.

‘Folic acid – vitamin B9 – helps a baby’s spine to develop and has been shown to have a protective role in the prevention of neural tube defects of the brain, spine or spinal cord,’ said Professor Sander.

‘The Department of Health recommends that all women take 0.4mg folic acid before and during their pregnancy, but women with epilepsy need to take a higher dose. This is because it is recognised that there is an increased risk of neural tube defects in children born to mothers taking anti-epileptic medications, particularly sodium valproate and carbamazepine.’

Researchers from the Wolfson Institute of Preventative Medicine at Queen Mary University of London say that as many as 2,000 cases of disabilities could have been avoided since 1998 in the UK, if flour had been fortified with the vitamin.

Their study, led by Professor Joan Morris, showed that failure to  add folic acid to flour had led to neural tube defects, often leading to stillbirths and terminations.

The US and 77 other countries already have a policy in place but currently the Department of Health recommendation in England is that women should take folic acid supplements before pregnancy to reduce their risk of having a baby with a serious defect.

The DOH is now considering the results of the study.

The policy of fortifying flour was introduced in the US in 1998 and has led to a 23 per cent fall in pregnancies with neural tube defects.

Folic acid is found in foods such as green vegetables, nuts and granary bread. It is added to some breakfast cereals but it can be difficult for women to get an adequate amount of the vitamin from diet alone.