Oncologists say vemurafenib, marketed by drugs firm Roche as Zelboraf, will transform the treatment of advanced malignant melanoma, after decades without a major medical advance.
One trial showed the twice-daily pill could increase survival, among those whose skin cancer had spread to other organs, by as much as eight to 10 months.
But doctors say equally exciting is that it makes patients feel so much better, reducing pain and giving them back their energy.
Every year, more than 11,000 people are diagnosed with malignant melanoma in Britain. Incidence is going up, partly because we take more foreign holidays.
Most tumours are removed before the cancer spreads. But for the fifth who are not that fortunate, the prognosis is usually very poor. Currently, only half survive longer than six months.
A large international trial showed vemurafenib boosted the proportion surviving to six months to 84 per cent, while another smaller study found median survival of 16 months.
Dr Paul Nathan, a consultant medical oncologist at the Mount Vernon Cancer Centre in Northwood, Middx., said: "This drug is a massive difference in what we have had before. We've never seen anything like it."
He had seen "major improvements" in the condition of patients who took it on a trial, he added.
Vemurafenib only works in about half of advanced skin cancer patients, in those whose melanomas have a mutated form of the BRAF gene, called V600. It works by turning this mutation off, retarding cancerous cells.
A small minority of patients have exhibited what is termed a "complete response", meaning physical tumours cannot be seen on CT scans, although the cancer appears to eventually develop resistance to the drug and return.
Vemurafenib is approved for sale across Europe as of today by the European Medicines Agency (EMA). Last December it recommended the drug for use, saying there was a "high unmet need" for treatments for people with advanced skin cancer.
Cally Palmer, chief executive of The Royal Marsden Hospital in London, said: "This is an important and significant step forward for the treatment of patients with advanced melanoma."
In England vemurafenib will have to be assessed by the National Institute for Health and Clinical Excellence (NIce) before it becomes widely available for NHS prescription. However, it is likely regional Cancer Drugs Fund boards will allow it before then.