A trial found that adding Herceptin to conventional chemotherapy delayed progression of the cancer by nearly three months compared with chemotherapy alone - an improvement of 46%.

Survival was extended from 20.4 months to 25.5 months on average. However, this result was not statistically significant.

Herceptin - the antibody drug trastuzumab - is only effective in women with the HER-2 gene, who account for around 20% of breast cancer patients.

In 2006, the NHS watchdog the National Institute for Health and Clinical Excellence (Nice) backed the use of Herceptin in early stage patients after coming under enormous pressure to make the drug more accessible.

But many doctors and health experts have questioned whether the NHS can afford the drug, which costs £20,000 per patient per year.

A team of researchers from Norwich said treatment for other cancer patients would have to be dropped to "balance the books".

The new research, presented at the annual meeting of the American Society for Clinical Oncology in Chicago, suggests that women on Herceptin benefit from continuing with the drug after their disease stops responding to traditional chemotherapy.

"It is important for patients and clinicians to know that trastuzumab keeps working in women whose aggressive HER 2-positive breast cancer comes back," said Dr Rob Stein, consultant oncologist at University College London Hospitals, who was involved in the study.

The international study involved 156 patients who were randomly assigned Herceptin together with the chemotherapy drug capecitabine, or capecitabine alone.