Doctors Panayiotis Zavos and Severino Antinori claim they are ready to embark on the greatest human experiment of our age. They say they will attempt to clone a human being before the year is out. Most people think the objections to this are ethical - human cloning would create many moral dilemmas.

There is another question that few ever ask: is the science actually ready yet for cloning healthy humans? Horizon follows the latest research, which has led many scientists to believe that Zavos and Antinori's plans to clone the first human could end in tragedy. The programme also meets couples like Matthew and Desiree Racquer who think cloning offers them the only way to raise a child who is truly their own.

For decades, cloning remained within the realms of science fiction. The idea that instead of combining a sperm and an egg, a new human could be made from a single cell taken from an adult, seemed completely absurd. But that all changed in February 1997, when the Roslin Institute introduced the world to Dolly the sheep - the first animal cloned from an adult. Ever since Dolly, scientists have been continuing to experiment with cloning animals. So far, they have succeeded in cloning sheep, cattle, pigs, goats and mice, fuelling the belief that humans could be next.

But even Dolly's creator, Professor Ian Wilmut, is concerned that beneath the veneer of success lies a disturbing reality. Most cloning attempts on animals so far have resulted in failed implantation or abnormal foetuses. Of the animals born alive, some soon die of catastrophic organ failure. Others appear to be healthy for weeks or even months, then die suddenly, sometimes from bizarre new illnesses which do not occur in nature.

Years of painstaking work are only now revealing some vital clues to what is going wrong. Horizon talks to the scientists who have uncovered new evidence, suggesting that the process of cloning itself causes subtle errors in the way genes function. These random errors may be like a time bomb inside every clone, causing some of the strange - often fatal - problems. There's no reason to think cloned human babies would fare any better. According to embryologist Dr Susan Avery, death might be the best outcome for many human clones. If they survived, they would suffer from catastrophic illnesses that modern medicine is powerless to prevent or cure.

Dr Zavos claims that these problems are the result of the still unsophisticated methods being used by animal researchers. Using advanced in vitro fertilisation ('test tube baby') techniques, he claims that he will strive to make human cloning safer than natural reproduction. Now though, it seems that some IVF procedures themselves are being investigated for possible harmful effects on the long term health of children. Professor Gerald Schatten of the University of Pittsburgh reveals evidence of these risks, which could be magnified in cloning.

Most reproductive specialists believe that the danger to any human born by cloning is enormous. But the would-be human cloners are determined to clone a human baby. If they proceed, they may be courting tragedy.