Patients whose cancer had spread to other parts of the body lived on average 22 months longer when docetaxel chemotherapy was added to standard hormone therapy.
Professor Malcolm Mason of Cardiff University, one of the senior investigators on the study, said he hoped the findings would lead to a change in medical practice.
“The type of chemotherapy, docetaxel, has already been in use in the NHS for many years in many types of cancer,” he said.
“In prostate cancer it has been used at a much more advanced stage of the illness, for some years – now we know that this chemotherapy should be added earlier, in fact as soon as hormone therapy starts.”
Until now, the standard treatment for men diagnosed with prostate cancer which has spread to other parts of the body was hormone therapy alone.
Professor Mason acknowledged that there could be cost implications but said there was “no alternative” because the results had been so beneficial.
Prostate cancer accounts for around one-fifth of all male cancers. In the UK there are around 25,000 new cases each year and around 10,000 deaths.
Lead study author Professor Nicholas James, Director of the Cancer Research Unit at the University of Warwick, said: “We hope our findings will encourage doctors to offer docetaxel to men newly diagnosed with metastatic prostate cancer, if they are healthy enough for chemotherapy.
“Men with locally advanced, non-metastatic prostate cancer may also consider docetaxel as part of upfront therapy, as it clearly delays relapse,” added Professor James, who is also a consultant in clinical oncology at Queen Elizabeth Hospital, Birmingham.
Peter Paul Yu, President of the American Society of Clinical Oncology (ASCO), said: “This is the biggest trial of its kind and strongly suggests that adding chemotherapy to standard hormone therapy can extend the lives of men with advanced prostate cancer.
“Its innovative design is exciting, and one that we may begin to see in other areas of oncology.”
This research is part of the STAMPEDE (Systemic Therapy in Advancing or Metastatic Prostate Cancer: Evaluation of Drug Efficacy) trial, the largest randomised clinical trial of treatment for men with prostate cancer ever conducted.
The STAMPEDE trial aims to prevent tumour re-growth by adding other treatment to hormone therapy.
More than 6,500 patients have been enrolled since 2005.
Velindre Cancer Centre in Cardiff has been one of the main participants in the research.
Professor Mason, one of the study designers, previously led a trial which showed that treating locally advanced prostate cancer patients with a combination of radiotherapy and hormone therapy halved the risk of dying from it.
The work helped change the way prostate cancer is treated.
In 2013, he was awarded a major scientific award for his research.
The study received funding and support from Cancer Research UK, UK Medical Research Council, the UK National Cancer Research Institute, the UK Department of Health, Sanofi-Aventis, Novartis, Pfizer, Janssen, and Astellas, University of Birmingham and University of Warwick.