Shares in GlaxoSmithKline fell after the drugs giant warned that trading this year depended on whether regulators increased competition for its biggest US product.

The UK pharmaceutical group said its sales beat forecasts last year coming in at £27.9bn ($34.8bn), 17% higher than a year ago.

But news that US regulators may allow generic competitors to its blockbuster asthma treatment Advair in the summer sent shares down more than 2% to 1,527p in London afternoon trading.

Glaxo said if regulators do not allow generic competition to Advair, it expects to enjoy core earnings per share growth of between 5% to 7% this year. But 2017 growth would be "flat to a slight decline" at constant exchange rates if competitors are introduced in the middle of this year.

The group's outgoing chief executive Sir Andrew Witty said: "Clearly, this year we face some uncertainty as to the level of our earnings performance, given the possibility of substitutable generic competition to Advair in the US, and this is reflected in the guidance we have issued today."

He added: "The next 24 months will be significant for GSK's pipeline and it marks the start of another intense period of R&D activity for the company, as we expect important data read-outs on around 20-30 assets in HIV, respiratory, immuno-inflammation, oncology and vaccines."

The business added that last year new product sales more than doubled to £4.5bn, partly driven by HIV drugs such Tivicay and respiratory treatments like Anoro.

Witty, who has served almost a decade as chief executive will be replaced later this year by Emma Walmsley, currently the head of the group's consumer healthcare business.