Scientists are claiming to have made a major breakthrough in the fight against prostate cancer with a new pill that shrinks tumors.

The new drug, abiraterone, could be used to treat up to 80 percent of patients with an aggressive form of prostate cancer, which almost always proves fatal.

The pill has been shown to minimize tumors and end the need for chemotherapy, which usually has unpleasant side effects and is not always effective.

Each year, 680,000 men worldwide are diagnosed with the disease and about 220,000 die from it.

In trials at the Institute of Cancer Research and the Royal Marsden Hospital in Britain, the majority of patients with previously untreatable and advanced cancer are said to have experienced "significant" regression of the disease.

Abiraterone, manufactured by Cougar Biotechnology Inc., is now being tested on 1,200 men worldwide.

"This agent clearly looks promising, but it is still at the early stages of clinical development," said David Webb, professor of clinical pharmacology at the University of Edinburgh.

"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," he said. "Important side effects often only emerge with the larger clinical studies that now need to be done."

Abiraterone works by blocking the generation of male hormones that drive growth of prostate cancers.

Cancer sufferer Simon Bush, 50, who took part in the trial at the Royal Marsden Hospital, told Sky News, "I had a feeling of complete hopelessness. Everyday that I woke up, I knew that things were not getting any better and the knowledge that more pain and discomfort is just around the corner is a frightening feeling.

"But abiraterone has transformed my life and given me and my family hope for the future," he added.

Click here for more on this story from Sky News.