Prof Seymour's innovative solution is to mask the virus from the body's immune system, effectively allowing the viruses to do what chemotherapy drugs do -- spread through the blood and reach tumours wherever they are.

"What we've done is make chemical modifications to the virus to put a polymer coat around it -- it's a stealth virus when you inject it," he said.

After the stealth virus infects the tumour, it replicates, but the copies do not have the chemical modifications. If they escape from the tumour, the copies will be recognised and mopped up by the body's immune system.

The therapy would be especially useful for secondary can cers, which spread in the body after the first tumour appears. "There's an awful statistic of patients in the west ... with malignant cancers; 75 per cent of them go on to die from these cells," said Prof Seymour.

Two viruses are likely to be examined in the first clinical trials: adenovirus, which normally causes a cold-like illness, and vaccinia, which causes cowpox and is also used in the vaccine against smallpox.