European Union farmers are banned from giving hormones to cattle because of health fears.

But US scientists give cattle both naturally occurring and synthetic hormones to boost the amount of top quality beef the animals produce.

By introducing female and male sex hormones into the animals, it is possible to increase the amount of meat that they produce without increasing the amount that they are fed.

By adding the female sex hormones oestrogen and progesterone to cattle, scientists can stimulate the animals to produce extra muscle and fat.

Adding the male sex hormone testosterone increases muscle growth, and decreases production of fat.

Oestrogen has been linked to an increased risk of breast cancer and to reproductive disorders in men.

Progesterone has been shown to increase the development of ovarian, breast and uterine tumours in laboratory animals.

And testosterone has been linked to prostate cancer in men.

Scientists also use synthetic compounds to mimic the action of the naturally occurring hormones.

Trenbolone acetate mimics testosterone and zeranol mimics oestrogen.

All these substances are implanted in tablet form under the skin of the animal, usually behind the ear or in an area that is unlikely to be eaten.

Once implanted, the tablets can be active for 50-60 days, slowly circulating the hormones around the animal's system.

Lingering concern

Scientists argue that because these substances have no effect if swallowed, then they should not pose any health risk to humans eating animals whose flesh has been treated in this way.

However, Professor Karl Linklater, a past president of the British Veterinary Association, said there is still concern over the safety of treating beef with hormones.

A similar chemical, stilboestrol, was used in the European Union until a ban on treating meat with hormones was introduced in 1988.

This led to reports of children growing breasts early after eating contaminated food.

The European Union is particularly concerned by a naturally occurring oestrogen known as oestradiol.

According to an EU report, even small residues of oestradiol in meat may produce tumours.

The EU scientific committee for vetinerary measures has warned that all sex hormones used in the US could pose a risk of cancer, genetic problems and brain disease, and claims children are most at risk.

However, the EU scientists admit that there is insufficient evidence to accurately assess the danger.

Professor Linklater said: "These things probably are safe, but they are unacceptable to the public because the perception is that they are not safe and people do not want to take any risk.

"You cannot eliminate the risk 100% and therefore you cannot say it is 100% safe."