GlaxoSmithKline bet another $212.9 million on the success of lung drug Relovair on Monday by raising its stake in U.S. biotech firm Theravance - its partner on the new medicine - to 26.8 percent.

Shares in the South San Francisco-based company jumped 15 percent to $22.40 on the news.

Britain's biggest drugmaker has been working with Theravance for 10 years to develop Relovair, an inhaled therapy combining two ingredients, which is a potential successor to GSK's $8 billion-a-year top-seller Advair.

Relovair will be submitted to regulators for approval as a treatment for chronic obstructive pulmonary disease (COPD) in mid-2012.

The new drug has had mixed results in clinical trials, showing superiority to Advair in some tests but not others. Still, GSK remains confident and hopes the convenience of once-daily dosing will appeal to patients, helping Relovair carve out good sales as a replacement for twice-daily Advair.

Current consensus forecasts point to annual Relovair sales of $1.46 billion by 2016, according to Thomson Reuters Pharma.

The success of Relovair is critical to GSK's future business in lung disease as Advair is set to lose patent protection in key markets, although it is not certain it will face immediate generic competition as respiratory drugs are difficult to copy.

GSK, which previously had an 18.3 percent holding in its U.S. partner, said it was paying $21.2887 per share for 10 million Theravance shares, a 7.5 percent premium to the five-day average price up to March 30.

(Reporting by Ben Hirschler; Editing by Keith Weir and Erica Billingham)