The American regulatory authorities are focusing on new treatments which use a de-activated cold virus, or adenovirus, to carry genes into the human body.

In September, an 18-year-old Arizona man, Jesse Gelsinger, died of liver problems following such treatment.

Last week drug manufacturer Schering-Plough said three others had suffered unspecified serious side effects.

There are also concerns surrounding the deaths of six people taking part in an trial of a gene therapy designed to help patients grow new blood vessels around blocked ones, although the doctors involve say they died of underlying illnesses.

But scientists carrying out gene therapy experiments in this country have stressed the safety of the viruses.

Some animals have been harmed by very high doses of adenovirus

Professor Stan Kaye, from the department of medical oncology at the Western Infirmary in Glasgow, is involved in a study in which viruses are injected into head and neck tumours.

He said: "I'm totaly confident of the safety of the virus.

"There is a genetic alteration which makes it harmless."

Professor David Kerr,a consultant in clinical oncology at the University of Birmingham, says the trial which ended in the death of Mr Gelsinger used huge doses of virus, whereas most other studies used them in comparatively tiny numbers.

Neutered virus

The starting dose his team are using contains approximately 10 million times fewer viruses.

He said: "The viruses have been neutered - we snip out the part of their genetic code which allows them to replicate."

Some scientists fear if a modified virus met a natural, or "wild" virus, a novel virus might be the result.

But despite the efforts of many, this had never been found to happen, even in the laboratory.

He said: "We are working with patients who have terminal cancer, perhaps only three to six months left.

"We are starting with very small doses and increasing them cautiously and carefully.

"We have done a lot of work with animal models, and although some rodents are susceptible to liver damage, the doses you have to give to achieve this are massive," said Mr Gelsinger.