GlaxoSmithKline said on Monday it planned to push ahead with filing its key new lung drug Relovair for regulatory approval in mid-2012, following the release of a mixed batch of clinical trials results.

Although the new once-a-day inhaled treatment did not show superiority to GSK's existing twice-daily drug Advair, Britain's biggest drugmaker said the totality of the data gave it confidence about the medicine's prospects.

The convenience of Relovair's once-daily dosing is expected to give it an important edge and GSK hopes it will carve out billions of dollars in sales as a replacement for Advair, which has worldwide sales of more than $8 billion (5 billion pound) a year.

But investors were underwhelmed by the details of the clinical studies and shares in GSK slipped 1.7 percent by 0815 GMT, underperforming a flat London stock market.

Relovair, which is being developed with U.S.-based Theravance , will be filed for chronic obstructive pulmonary disease (COPD) in the United States and Europe in the middle of the year.

It will also be submitted as a new treatment for asthma in Europe at the same time, while in the United States GSK will continue discussions with regulators on its use in asthma.

If all goes well, industry analysts believe Relovair should reach the market in 2013 and consensus forecasts point to sales of $1.88 billion by 2016, according to Thomson Reuters Pharma.

Darrell Baker, Glaxo's head of respiratory medicine development, said Relovair had reached an important milestone in its development.

Having undertaken an initial assessment of these data we believe they support our plan to seek global approvals of this once-daily medicine for the treatment of patients with COPD and asthma, he said in a statement.

GSK has previously built a 19 percent stake in Theravance in a vote of confidence of its respiratory collaboration with the U.S. firm.

Advair is set to lose patent protection in key markets but it is not certain that it will face generic competition immediately as respiratory drugs are notoriously difficult to copy.

Both Advair and Relovair are inhaled drugs that combine a steroid and a bronchodilator to open the airways. Bronchodilators provide short-term relief while steroids have a longer-term impact.

(Reporting by Ben Hirschler; Editing by Hans-Juergen Peters)