An experimental drug that extends the lives of lung cancer patients by a third has impressed doctors, who say that it is one of the first signs of progress against the disease.

Lung cancer is among the most lethal of all cancers and claims more lives each year than any other. Only five per cent of patients survive more than five years.

Efforts to find an effective treatment have failed and unlike other cancers, such as breast cancer, the death rate has not changed for more than a decade. A trial of the as yet unnamed drug, known as AS1404, in 70 patients with non-small cell lung cancer (NSCLC), the commonest kind, found they lived for 14 months on average compared with 8.8 months for those given chemotherapy alone.

Although the extra five months gained is small in absolute terms, it is a significant improvement, suggesting a genuine benefit from the drug. More than 26,000 people in Britain die from NSCLC each year.

The drug, discovered by researchers in New Zealand and developed by the UK biotechnology company, Antisoma, with backing from Cancer Research UK, is one of a new class of compounds called vascular disrupting agents. These work by destroying blood vessels that supply solid tumours on which the tumours depend to survive and grow.

The success of the phase II study lays the ground for a larger phase III trial, the outcome of which will determine whether the drug is licensed as safe and effective. AS1404 has already been shown to be effective in phase II trials of prostate and ovarian cancer.

Mark McKeage of the University of Auckland, one of the researchers, said: "It is great to see this large survival benefit with AS1404 in lung cancer patients. This makes me feel very optimistic as we progress into phase III testing."

Professor Alex Markham, chief executive of Cancer Research UK, said: "Our drug development team played an integral role in the early development of the drug and we're delighted with this news. We look forward to seeing how the drug performs in a much larger number of patients."

The charity has set up a company to raise the number of undeveloped new treatments by putting them into clinical trials. Since 1982, more than 100 agents have been put into trials.

"Over the next five years we plan to double our activity and speed up the drug development process, getting even more new drugs into clinical trials," Professor Markham said.

* Living near heavy industry may raise the risk of lung cancer. A study of women in Teesside, who lived within three miles of a chemical production complex at Billingham for at least 25 years, found they were twice as likely to develop the disease than others, after smoking and other factors were taken into account.