Around 20,000 men a year with prostate cancer could benefit from a combination of drugs that boost survival dramatically, experts say.
A clinical trial run by Cancer Research UK - believed to be the biggest cancer treatment trial in the world - has found that giving two therapies at once cuts disease progression and offers some patients the chance of a cure.
Researchers say the new drug regime could "transform the treatment" of 20,000 men newly diagnosed with the disease each year in England.
Of these, 5,000 men with the most advanced disease which has spread around the body could see their life expectancy jump from 3.5 years to seven years on average.
Of the 15,000 diagnosed when the disease is confined to the pelvic area, most could expect to live as long as they would if they were cancer-free.
The study, presented at the American Society of Clinical Oncology (ASCO) meeting in Chicago, included around 1,900 men with locally-advanced cancer or whose disease had already spread.
Half were given the standard hormone therapy treatment known as androgen-deprivation therapy (ADT) while the other half received ADT plus the drug abiraterone.
Usually, abiraterone is given to men who have stopped responding to ADT but the study found that giving it much earlier - and in combination with ADT - had much stronger benefits.
Adding abiraterone to ADT reduced the risk of dying over three years by 37% compared with men just on ADT.
It also lowered the chance of the cancer getting worse by 71% compared with men just on ADT.
After three years, 83% of men in the abiraterone group were still alive compared with 76% on standard ADT.
Abiraterone could now become a first-line treatment alongside ADT for men with advanced prostate cancer.
The National Institute for Health and Care Excellence (Nice), which approves drugs for the NHS, is already looking at the findings.
Professor Nicholas James, chief investigator of the trial, from the University of Birmingham, said some patients may effectively be cured while others could expect to live as long as they normally would.
He added: "These are the most powerful results I've seen from a prostate cancer trial – it's a once in a career feeling. This is one of the biggest reductions in death I've seen in any clinical trial for adult cancers.
"Abiraterone is already used to treat some men whose disease has spread but our results show many more could benefit.
"In addition to the improvements in survival and time without relapse, the drug reduced the rates of severe bone complications, a major problem in prostate cancer, by more than a half.
"I really hope these results can change clinical practice."
The men in the trial, known as Stampede, were aged 67 on average at the start of the study.
Some 52% had disease that had spread around the body while 48% had cancer confined to the prostate or pelvis.
After three years, fewer than 5% of those on abiraterone whose disease was confined had relapsed and got worse compared to 25% in the ADT arm of the trial.
Sir Harpal Kumar, Cancer Research UK's chief executive, said: "These results could transform the treatment of prostate cancer. Abiraterone can clearly help many more prostate cancer patients than was first thought."
Alfred Samuels, 59, was diagnosed with advanced prostate cancer in January 2012 and joined the trial two months later. He lives in Harrow with his partner and five children.
He said: "It felt like my world fell apart overnight. The doctors explained that surgery wasn't an option for me because the cancer had spread beyond my prostate.
"As part of the trial, I started taking abiraterone four times a day and had a hormone injection every eight weeks."
Tests showed the treatment is working and his cancer is being well-managed.
Each year, around 46,500 men are diagnosed with prostate cancer in the UK, and around 11,000 men die from the disease.
Around half of the men diagnosed are eligible for hormone therapy, which blocks the action of male sex hormones.
Abiraterone goes further and shuts down production of the hormones that fuel prostate cancer's growth.
Dr Iain Frame, director of research at Prostate Cancer UK said: "The potential benefits of giving some men abiraterone alongside hormone therapy are clearly impressive and we will be working with all relevant bodies to make sure this treatment becomes an option available for these men via the NHS."
The results of the trial are published in the New England Journal of Medicine.
Professor Paul Workman, chief executive of the Institute of Cancer Research, London, said: "Abiraterone has already hugely improved the outlook for hundreds of thousands of men with advanced prostate cancer worldwide.
"Now, the new results tell us that patients just starting hormone therapy can also benefit from this drug, which was discovered here at the ICR.
"Abiraterone is a highly innovative treatment that not only improves survival rates but has lower rates of side-effects than conventional therapies.
"I'm keen to see abiraterone assessed by Nice for use in patients as early as possible in their treatment, to maximise the benefit that people get from taking it."