Expectant mothers have been warned that they could be harming their unborn child by taking multivitamins.
Too much vitamin A can damage babies growing in the womb
Trading Standards watchdogs and charity Birth Defects Foundation Newlife found a third of products do not carry clear labels showing they contain vitamin A.
Too much of this vitamin can interfere with organ formation in the growing foetus and therefore supplements should be avoided during pregnancy.
Mothers-to-be were told to heed the advice from day one of pregnancy.
Professor Michael Patton, medical director of BDF Newlife and consultant clinical geneticist at St George's Medical Hospital School in London, said: "Women who are pregnant or likely to become pregnant should avoid supplements of Vitamin A or multivitamin tablets containing Vitamin A, as this may cause damage to the developing baby in the womb."
The warnings do not apply to multivitamin tablets specifically designed for pregnant women.
The charity and Oxfordshire County Council's Trading Standards, who carried out the research, are calling for a vitamin A warning to be made compulsory on all multi-vitamin products.
Sheila Brown of BDF Newlife said: "Ensuring good labelling with specific warnings about Vitamin A on all vitamin products, whether these are general vitamins or those specifically marketed at pregnant women, is essential.
"Most women with a good balanced diet do not need to take a supplement containing Vitamin A and should only do so if advised by a doctor or antenatal clinic."
Likewise they should avoid Vitamin A rich foods, such as liver, she said.
"This message needs understanding before conception so they can avoid using certain products from day one of becoming pregnant.
"Ensuring the public and particularly women of child-bearing age know this, is a key message in the prevention of inborn conditions," she said.
The researchers looked at 60 multivitamin products on sale between November 2004 and March 2005.
Twenty of the 60 did not carry vitamin A warnings. These included big brands like Sanatogen, Centrum and Quest. Boots and Tesco products did carry warnings. Bayer Healthcare, which owns the Sanatogen brand, said that a number of their multivitamin products - those not specifically targeted at pregnant women - currently did not contain a specific warning about vitamin A.
In a statement, the firm said: "Bayer Consumer Care agrees with the report that good labelling for all vitamin products is essential, whether these are general vitamins or those specifically marketed at pregnant women."
A spokesperson for the Proprietary Association of Great Britain, which represents manufacturers of medicines and food supplements sold over-the-counter in the UK, said: "With regards to vitamin A, our member companies are waiting for Europe to announce recommendations for labelling and usage of vitamin A in food supplements.
"To ensure the safety of the newborn child, anybody who is pregnant or planning a pregnancy should always discuss any medication and use of food supplements with their doctor or midwife."