Early treatment with a chemotherapy drug extends the lives of patients with advanced prostate cancer by nearly two years, a major study shows.
Docetaxel is normally given after hormone treatment has failed.
But results, to be presented at the American Society of Clinical Oncology, will show earlier treatment can extend life expectancy from 43 to 65 months.
Experts said the findings from a trial in Britain and Switzerland were "potentially game-changing".
More than 40,000 men are diagnosed with prostate cancer and nearly 11,000 die in the UK each year.
During the trial, being run across Britain and Switzerland, 2,962 men took part in the trial and some were given six doses of docetaxol at the start of their treatment.
Overall, patients who received the drug lived 10 months longer, but for patients where the cancer had already spread beyond the pelvis, the increase in life expectancy was 22 months.
Prof Nicholas James, one of the researchers at Warwick University, called for all patients with prostate cancer that had spread to be given docetaxel when they are diagnosed.
He said the NHS needed to act quickly: "To see a 22-month survival advantage off six lots of treatment given several years earlier is a very big benefit. We are very pleased by it."
Fellow researcher Prof Malcolm Mason, from Cardiff University, added: "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."
It would be relatively cheap to do as docetaxol is out of patent.
John Angrave, 77, from Hinckley in Leicestershire, was told that he had three to five years to live.
That was seven years ago.
He said: "I am alive. I have a good quality of life and I am alive.
"I walk. I go fishing. I can spend time with my great-grandchildren."
The researchers say they need to monitor patients for longer to see if the drug significantly prolongs life if the cancer has not spread.
There were side effects from the treatment, but the doctors said they were "manageable".
Cancer Research UK said the results were "important" and "show that it should be given earlier in a man's treatment".
Dr Iain Frame, the director of research at Prostate Cancer UK, said: "The findings of this trial are potentially game-changing - we can't wait to see the full results.
"Chemotherapy is currently one of the last-resort treatments for advanced prostate cancer.
"If it is shown to have a much greater impact on survival when prescribed earlier and alongside hormone therapy, that's incredibly exciting, and we would want to see this brought in to the clinic so it can benefit men without delay."
The study is one part of a much the wider Stampede trial which is assessing the impact of using a range of drugs or radiotherapy in conjunction with conventional hormone therapy.