LONDON (Reuters) - GlaxoSmithKline (>> GlaxoSmithKline plc) said on Thursday that its chairman-designate Philip Hampton would take charge of Britain's biggest drugmaker from May 7, an earlier transition than initially anticipated.

Hampton, who chairs Royal Bank of Scotland (>> Royal Bank of Scotland Group plc), was named as the pharmaceutical company's next chairman last September, but GSK said at the time he might not take up the position until Sept. 1, 2015, if he could not be released from other commitments.

In the event, Hampton is now able to take over with effect from the drugmaker's next annual meeting, following the appointment of Howard Davies as his successor at RBS.

Hampton is taking over at a testing time. GSK was hit last year by a record fine of nearly $500 million in China for bribing doctors and has disappointed investors with weak sales of its core respiratory drugs.

The poor performance was reflected in a big cut in the bonus paid to Chief Executive Andrew Witty, according to the company's annual report released on Thursday. Witty was awarded a bonus of 917,000 pounds for 2014, a reduction of 51 percent compared to 2013.

GSK is hoping that a $20 billion-plus asset swap with Novartis (>> Novartis AG), which is due to close next week, will help revive its fortunes, although the deal promises a long-term path to more sustainable growth rather than any short-term fix.

Hampton will replace Chris Gent, who has chaired GSK since 2005, having previously led mobile phone company Vodafone (>> Vodafone Group plc) during a period of rapid growth.

Some analysts believe Hampton could usher in a period of significant change at GSK. Witty has already suggested that the company could consider spinning off some component units.

Hampton has been chairman of RBS since 2009, when he was parachuted in to help rescue the bank following its 45 billion- pound bailout during the financial crisis.

He has led RBS through a turbulent period during which it shed assets worth 1 trillion pounds to rid itself of toxic loans.

Hampton joined GSK's board on Jan. 1 and will become deputy chairman of the group on April 1.

(Editing by Greg Mahlich/Ruth Pitchford)

By Ben Hirschler