Abiraterone could potentially treat up to 80% of patients with a deadly form of the disease resistant to currently available chemotherapy, they say.
The drug works by blocking the hormones which fuel the cancer.The Institute of Cancer Research hopes a simple pill form will be available in two to three years.
An advanced clinical trial involving 1,200 patients around the world is currently under way, with more trials likely later this year.
Prostate cancer is the most common cancer among men.
It is estimated that up to 10,000 men a year in the UK are diagnosed with the most aggressive - and almost always lethal - form of prostate cancer.
Typical life expectancy following chemotherapy is no more than 18 months.
It had been assumed that the cancer was driven by sex hormones such as testosterone produced in the testicles.
Current treatments work by stopping the testicles from producing testosterone.
However, experts have now discovered that the cancer can feed on sex hormones from all sources, including supplies of the hormone produced by the tumour itself.Abiraterone works by blocking production of the hormones throughout the body.
The latest study, published in the Journal of Clinical Oncology, is based on just 21 patients with advanced, aggressive prostate cancer treated with the drug - but data has been collected on a total of 250 worldwide.
It found significant tumour shrinkage, and a drop in tell-tale levels of a key protein produced by the cancer called prostate specific antigen in the majority of patients.
Many of the patients have reported a significant improvement in the quality of their lives.
Some were able to stop taking morphine for the relief of pain caused by the spread of the disease to their bones.
Lead researcher Dr Johann de Bono said the findings needed to be confirmed in larger trials.
At this stage, no patient has taken the drug for longer than two-and-a-half years, and so it has not been possible to determine exactly what the effect of the drug on life expectancy will be.
But he said: "We believe we have made a major step forward in the treatment of end-stage prostate cancer patients.
"These men have very aggressive prostate cancer which is exceptionally difficult to treat and almost always proves to be fatal.
"We hope that abiraterone will eventually offer them real hope of an effective way of managing their condition and prolonging their lives."
It is hoped the drug will also aid other cancer patients, including those with breast cancer.
Professor David Webb, an expert in clinical pharmacology at the University of Edinburgh, said: "This agent clearly looks promising, but it is still at the early stages of clinical development.
"It will be crucial to look carefully at the balance between its benefits and harms, before drawing firm conclusions about the usefulness of this new drug.
"Important side effects often only emerge with the larger clinical studies that now need to be done."
John Neate, of The Prostate Cancer Charity, said: "This is an exciting development which has been eagerly anticipated."Found this Post interesting? Discover more Curious Reads.