Doctors say they have used a genetically engineered herpes virus to treat successfully patients with head and neck cancer.
A London hospital trial of 17 patients found that use of the virus alongside chemotherapy and radiotherapy helped kill the tumours in most patients.
It works by getting into cancer cells, killing them from the inside, and also boosting the patient's immune system.
Further trials are planned for later in the year.
Head and neck cancer, which includes cancer of the mouth, tongue and throat, affects up to 8,000 people every year in the UK.
Study leader Dr Kevin Harrington, who is based at the Institute of Cancer Research in London, said current treatments were effective if the cancer was picked up early but that many patients were not diagnosed until it was more advanced.
The herpes virus, which is also being tested in patients with skin cancer, is genetically manipulated so that it grows inside tumour cells but cannot infect normal healthy cells.