Millions of people with breast cancer could safely avoid chemotherapy as scientists have developed a DNA test that can distinguish between patients who are likely to benefit from the treatment and those who are not, according to the results of a trial.

The international study found that more than two-thirds of its participants could be spared the side of effects of chemotherapy and be treated with hormone therapy alone.

Chemotherapy can cause fatigue, nausea, hair loss, a weakened immune system and fertility issues.

The study, led by University College London (UCL), involved more than 4,000 newly diagnosed patients over the age of 40 in the UK, Norway, Sweden, Australia, New Zealand and Thailand.

Scientists used a gene test called Prosigna to measure the activity of 50 genes involved in breast cancer growth and calculate a patient's risk of the disease returning.

Those who received a low score - two-thirds of the group - were not treated through chemotherapy. The five-year survival rate of their group was 93.7%, compared with a 94.9% rate among patients who received chemotherapy as part of their care.

The primary treatment for breast cancer is usually surgery to remove tumours. Chemotherapy is often recommended afterwards to diminish the risk of return.

It is also regularly offered to people with early-stage breast cancer that has spread to the nearby lymph nodes.

Clinicians are concerned the treatment provides little benefit to those with the most common type of breast cancer, UCL said.

The university said more than 5,000 NHS patients a year could avoid chemotherapy as a result of the trial.

Karen Bonham, from Cardiff, took part in the trial and said the results are an "immense relief" and feel "like Christmas".

The 64-year-old avoided chemotherapy thanks to the Prosigna test and has instead received eight years of radiotherapy and hormone therapy.

"Cancer diagnosis and treatment can be shocking," she said.

"It certainly propels you into a world of uncertainty. Life priorities realign - you simply want to survive."

The findings of the study will be presented at the world's largest cancer conference, the American Society of Clinical Oncology's annual meeting, in Chicago, United States, on Saturday.

The trial's chief investigator, and professor of breast oncology at the UCL Cancer Institute, Professor Rob Stein, said: "These results mark an important and significant step toward more personalised treatment.

"The trial has successfully used tumour biology to guide decisions rather than relying solely on traditional clinical features.

"For patients, this means many may be spared the physical and emotional burden of chemotherapy and its potential long-term side effects.

"For health systems, it represents a more efficient and evidence-based use of resources."

It is not known whether the findings apply to people under the age of 40, with a result still several years away, according to UCL.