Andrew Witty, the chief executive-designate of GlaxoSmithKline, pledged to diversify the UK pharmaceuticals group as its shares dropped after an earnings warning.

GSK shares tumbled 7.6 per cent to £10.78, as outgoing chief executive Jean-Pierre Garnier said last year's safety scares over Avandia, the group's diabetes treatment and its second biggest-selling drug - which wiped $1bn off profits - and further "erosion" of revenues by generics would contribute to a "mid-single digit percentage decline" in earnings per share, worse than analysts had expected.