But it could soon benefit even more sufferers. A global study of 500 patients whose cancer cells grow more slowly also significantly extended their survival, saving them precious extra months with loved ones. The drug reduced the chance of cancer progression by 49% and the risk of death during the study by 36%, compared to chemotherapy alone.

It nearly doubled the time the tumors remained stable or shrunk from an average of 5.4 months to 10.1 months. And overall survival went from 17.5 months to 23.9 months.

The results suggest that up to 16,800 people in the UK with breast cancer could benefit.

The National Institute for Health and Care Excellence said it would consider the results when reassessing the drug.

Breast cancer patients are currently divided into two categories based on their levels of a protein called HER2, which accelerates tumor growth.

Those with high levels are HER2-positive and those with normal or low levels are HER2-negative.

Enhertu trials are currently underway to test the drug in low HER2 patients with early stage breast cancer. If successful, it could offer new treatment options to tens of thousands of women and even increase the number of people whose cancer is cured, it has been said.

Dr Susan Galbraith, of Enhertu maker AstraZeneca, said: “This is a historic breakthrough and it’s a watershed moment as it reclassifies breast cancer – it’s a fundamental change.”

Dr Galbraith said the discovery was “very exciting” and that Enhertu could be available to this new group of early stage breast cancer patients on the NHS within two years.

The findings were presented at the American Society of Clinical Oncology’s annual conference in Chicago yesterday.

Jane Lowe Meisel, ASCO breast cancer expert at Emory University School of Medicine, Atlanta, said: “By effectively creating a new class of breast cancer, HER2-low, this trial will redefine the way we classify cancer. breast cancer and will significantly expand the patient population that can benefit from HER2-targeted therapy.

Dr Kotryna Temcinaite, from the charity Breast Cancer Now, said: “It is extremely exciting that Enhertu can benefit even more breast cancer patients, giving them hope to spend time extra precious with their loved ones.

“This treatment now needs to be rapidly submitted for authorization and assessed for use on the NHS, so that this different group of eligible women have the chance to benefit from it as soon as possible.”

Professor Charles Swanton, of Cancer Research UK, said: “This study could form the basis for future approvals to extend its use to more patients.”