Patients will be recruited within the next two months and will be given seven injections under the skin over a five-month period, during which time doctors will assess their immune response to the vaccine and test them for any sideeffects.

Dr Reiner Laus, president of the drug company, said the vaccine would have far milder side effects than existing treatments like chemotherapy and would hopefully be licensed by 2015.

He added: “Right now this is the only cancer vaccine in this late stage of development with this large-scale potential for patients to benefit from it. Our testing stage is the last and most advanced.”

Results from earlier stage trials, which are primarily designed to test safety, suggest it could extend patients’ lives by an average of nine months, compared with four months after chemotherapy.

Because the vaccine, known as Prostvac, has been genetically modified researchers had to seek approval from the Department for Environment, Food and Rural Affairs, which assesses potential environmental and human health risks, as well as medical regulators for the trial to begin.

Officials gave the go-ahead in January after finding there was only a “minimal risk” of the modified virus escaping into the environment or infecting healthy people.

Trials have already begun on patients at various locations in America, and are planned to take place in 18 other countries as well as the UK.

The therapy is a combination of the virus used in a smallpox vaccine and a strain of the fowlpox virus, which causes diseases in poultry.

Scientists added four human genes to the DNA of the viruses which caused them to develop chemical signals which are found in cancerous cells but not healthy ones.

By delivering the signature protein, known as prostate-specific antigen (PSA), in the form of a virus the treatment provokes the immune system into eliminating any cells which carry it.

Dr Laus said the vaccine could potentially be adapted in future to form a protective effect.

He said: “It is now being tested in patients who already have prostate cancer, as a treatment rather than a prevention, but one of the attractions with a therapy like this is we can move it to earlier stages, and maybe eventually use it preventively.

Dr Kat Arney of Cancer Research UK said: “Many researchers around the world are investigating the potential of vaccines for treating cancer.

“Early trials of this vaccine have been promising, so we look forward to seeing the results of this larger trial in men with advanced prostate cancer, a disease that urgently needs more effective therapies.”

Dr Kate Holmes, Head of Research at The Prostate Cancer Charity said: “This vaccine will not prevent prostate cancer, however results to date show it may be an exciting breakthrough for the treatment of men in the final stages of the disease.

“Men with advanced prostate cancer currently have very few treatment options available to them and if proven successful in the third trial phase this new vaccine would offer these men an urgently needed additional life extending treatment once other available options have stopped working. We await with anticipation the results of the phase III trial.”