Drugmaker GlaxoSmithKline grew revenue just 1 percent in the first quarter, reflecting pressures on its business from government price cuts in Europe and some emerging markets and tough year-ago comparisons.

The performance was flattered by the sale of U.S. rights to a bladder drug, which added some 170 million pounds ($275 million) to turnover.

GSK - which is trying to buy its long-time biotech partner Human Genome Sciences for $2.6 billion - is emerging from a trough caused by patent expiries and collapsing sales of diabetes pill Avandia, linked to heart risks.

Chief Executive Andrew Witty reiterated GSK was on track to return to sales growth in 2012, after a difficult few years, with gradually improving margins.

Quarterly sales were 6.64 billion pounds, generating core earnings per share (EPS) up 5 percent at 27.3 pence, Britain's largest pharmaceuticals company said on Wednesday.

Analysts, on average, had forecast sales of 6.83 billion pounds and core EPS of 29.1p, according to Thomson Reuters I/B/E/S Estimates. Core earnings exclude certain non-cash charges.

Witty has been diversifying the group to reduce reliance on white pills in Western markets, the part of the business most vulnerable to generic competition and price cuts imposed by cash-strapped governments.

It is now over the worst of its patents losses, although there is uncertainty about when top-selling lung drug Advair will face generic competition. Some analysts also question whether follow-on medicine Relovair can fill the gap, after it produced mixed results in clinical trials. ($1 = 0.6192 British pounds)

(Reporting by Ben Hirschler; editing by Clara Ferreira-Marques)