Patients with aggressive skin cancer have been treated successfully using a drug based on the herpes virus, in a trial that could pave the way for a new generation of cancer treatments.

The findings mark the first positive phase 3 trial results for cancer "virotherapy", where one disease is harnessed and used to attack another. If approved, the drug, called T-VEC, could be more widely available for cancer patients by next year, scientists predicted.

Crucially, the therapy has the potential to overcome cancer even when the disease has spread to organs throughout the body, offering hope in future to patients who have been faced with the bleakest prognosis.

Kevin Harrington, professor of biological cancer therapies at the Institute of Cancer Research London, who led the work, said: "This is the big promise of this treatment. It's the first time a virotherapy has been shown to be successful in a phase 3 trial."

In the trial, involving more than 400 patients with aggressive melanoma, one in four patients responded to the treatment, and 16% were still in remission after six months. About 10% of the patients treated had "complete remission", with no detectable cancer remaining - considered a cure if the patient is still cancer-free five years after diagnosis.

The results are especially encouraging, Harrington said, because all the patients had inoperable, relapsed or metastatic melanoma with no conventional treatment options available to them. "They had disease that ranged from dozens to hundreds of deposits of melanoma on a limb all the way to patients where cancer had spread to the lungs and liver," he said.

The treatment works by mounting a two-pronged attack on cancer. It is based on a genetically "neutered" version of the herpes virus, which has been modified to stop it producing the protein that allows it to infect healthy cells. Cancer cells then produce their own version of the blocked protein, filling in the deficit and allowing the modified virus to thrive within cancerous tissue.