The report by the Scientific Advisory Committee on Nutrition (SACN), is published on the same day that the Food Standards Agency launches a consultation setting out options for improving the intake of the vitamin folate for young women.

The mandatory addition of folic acid to bread or flour is one of the four options being considered by the agency.

The SACN said it as the most effective way of protecting pregnant women against neural tube defects.

Studies have shown that adequate levels of folic acid in the diet reduce neural tube defects and women who are planning to get pregnant are currently advised to take supplements of folic acid.

Countries such as the USA, Canada and Chile, have already introduced the mandatory fortification of flour and reported a decrease in the number of pregnancies affected by neural tube defects from 50 per cent to 27 per cent.

A year ago the SACN concluded that of the 600 pregnancies a year it estimated were affected by such defects, 41 per cent could be prevented by fortification of flour.

However, around half of all pregnancies are unplanned and mothers need to start taking folic acid supplements before they fall pregnant in order to gain the full benefits.

Previous studies have raised concerns that folic acid masked signs of vitamin B12 deficiency in the elderly, which in severe cases can cause neurological damage.

However, the SACN said there is no evidence that this occurs when folic acid doses are 1mg per day or less. There are also no reports from countries which already have mandatory fortification.

The FSA will take account of SACN's report plus responses received during the 13-week consultation before advising health ministers on the issue next year.

Others options being considered are: to continue the current policy of advising women to take folic acid supplements before conception and during early pregnancy; stepping up efforts to encourage young women to eat more folate; and encouraging manufacturers to voluntarily fortify more foods with folic acid.

Rosemary Hignett, head of nutrition at the FSA, said: 'Taking action to improve the folate status of young women, in order to help prevent neural tube defects such as spina bifida occurring during pregnancy, is a particularly complex one.

"Over the past few years, this issue has raised many interesting and important issues.

"The Food Standards Agency is committed to policy-making that will benefit people's health and we do this on the basis of weighing up the evidence in relation to risks and benefits."

In July 2002 the FSA advised health ministers not to introduce mandatory fortification at that time due to lack of available evidence about possible risks and benefits.

The SACN recommended that mandatory fortification should only be introduced in the UK if it is accompanied by long-term monitoring. And said that measures to ensure consumers do not eat too much folic acid in the long term via other fortified foods and supplements would also be needed.

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