The brokerage, which raised its price target on GSK stock on Monday, forecast combined risk-adjusted annual sales for dabrafenib and trametinib of 1.5 billion pounds ($2.35 billion)by 2020, some three times consensus expectations.

Dabrafenib, designed to work in patients with a mutation of a gene known as BRAF, and trametinib, which interferes with a protein known as MEK, have been shown in small-scale tests to curb melanoma with few side effects.

Further details on GSK's products will be disclosed at the American Society of Clinical Oncology annual meeting in Chicago next week.

Citi analyst Andrew Baum said dabrafenib was likely to be launched in 2013 and would quickly erode sales of Roche's Zelboraf, given it had fewer skin side-effects and less joint pain, while from 2014 a combination of dabrafenib and trametinib was likely to become the "gold standard" for melanoma treatment.

Roche's Zelboraf, or vemurafenib, is the only BRAF inhibitor approved for treating melanoma, but the Swiss group is further behind GSK in developing MEK inhibitors.

GSK's melanoma medicines are forecast to generate sales of 800 million pounds in patients with advanced disease, and 700 million in potentially curable patients, by 2020, Citi said.

The brokerage, which rates GSK shares a "buy", raised its target to 16.10 pounds from 15.70, reflecting growing confidence in the company's pipeline of new drugs.

GSK stock was 1.5 percent higher at 14.39 pounds by 1030 GMT (6:30 a.m EDT), while the FTSE 100 index rose 1 percent.

Britain's biggest drugmaker has made oncology a priority for drug development but has struggled to achieve significant sales, with Tykerb for breast cancer, in particular, failing to meeting early lofty sales forecasts.

($1 = 0.6396 British pounds)

(Reporting by Ben Hirschler; Editing by David Hulmes)

• A Permanent Economic Emergency