CHILDREN born using ICSI - an in-vitro fertilisation technique that allows infertile men to be fathers - may have slower mental development than children conceived naturally, research being published today suggests.

The findings, in the Lancet, are based on a study in Australia and indicate that ICSI children have a significantly greater than average chance of suffering delayed mental development at one year old.

ICSI, or intracytoplasmic sperm injection, entails taking an individual sperm in a glass injector and injecting it into an egg. In this way a man with a low sperm count can be given a much higher chance of fathering.

The technique, developed in Belgium, was hailed as a breakthrough when introduced in 1993 and has since been widely adopted at clinics all over the world. The first Scots children conceived in this way were born in 1995 after the procedure was performed at Aberdeen Royal Infirmary.

Tens of thousands of children are now thought to have been born worldwide as a result of ICSI. But some critics have warned that little is known about the possible long-term effects on children conceived in this way.

Researchers led by Dr Jennifer Bowen, from the Department of Neonatology, Royal North Shore Hospital, St Leonard's, New South Wales, compared 89 ICSI children with 84 children born after routine In-Vitro Fertilisation and 80 who were conceived naturally.

There were no physical differences between them, but 17% of the ICSI children showed ''mildly or significantly delayed development'' at one year old. This compared with just 2% for the traditional IVF children and 1% for those who were naturally conceived.