DOCTORS have uncovered the first evidence that fathers of test-tube babies may be passing on their infertility to their sons.

A new study has found that boys conceived through IVF treatment involving a single sperm being directly injected into a female egg often inherit shorter fingers, a trait known to be associated with infertility.

The results raise the prospect of a new and growing generation who may be less likely to have children of their own.

There are now an estimated 1m children across Europe born through IVF treatment.

Almost one in 50 British babies is conceived artificially and nearly half the couples having treatment go through a procedure known as ICSI (intracytoplasmic sperm injection).

The technique bypasses the normal competition where only the healthiest sperm cell is able to reach the female egg and fertilise it.

Alastair Sutcliffe, a paediatrician at the Institute of Child Health in London, led the Anglo-German study which compared 211 six-year-olds conceived through ICSI with 195 naturally conceived children of the same age.

The ICSI group were similar heights to the naturally conceived group, but the boys had significantly shorter fingers.

It is known that men with low sperm counts often have ring fingers the same length as their index finger, whereas fertile men are more likely to have a ring finger which is relatively longer than their index finger.

The effect is reversed in women, where the most fertile are likely to have index fingers significantly longer than their ring fingers.

Sutcliffe;s findings appeared recently in the journal Reproductive Biomedicine Online.

"This is the first study of this kind on these children," Sutcliffe said.

"We don't yet know the implication of the findings because the children are very young, but we need to inform people [about the possible risks of the ICSI procedure]."

Scientists have long suspected that the test-tube baby boom would bring its own problems. Infertility treatment began as a commercial operation only in the 1990s. The first ICSI baby was born in 1992 and there are now about 3,700 such births a year in Britain.

Finger length is known to be set within the first 14 weeks of pregnancy and is linked to testosterone exposure which is, in turn, governed by a specific group of genes.

"This [research] is telling us that we should only use ICSI when it is absolutely necessary," said John Manning, an evolutionary biologist at Southampton University who has examined the link between finger length and fertility and who is one of the authors of the latest study.

"We know the extraordinary depression and pain that childlessness can cause and we have a responsibility to ensure that the focus on the wellbeing of the children born as a result of these techniques is as high as it can be."