GlaxoSmithKline has revealed its chief executive Sir Andrew Witty will step down earlier than expected in 2017 after bowing to pressure from institutional shareholders.

The pharmaceutical giant said Witty, who has been in charge of the FTSE 100 group since 2008, will retire on 31 March next year and added a research for his successor will begin immediately. GSK's outgoing CEO has come under intense pressure from shareholders, as the firm has endured disappointing sales in its US market and a lagging share performance.

GSK shares have returned an average of 10% a year over the past five years, compared with a 17% average annual return for the Bloomberg Europe Pharmaceutical Index.

"By next year, I will have been CEO for nearly 10 years and I believe this will be the right time for a new leader to take over," Witty said in a statement, while GSK indicated both external and internal candidates will be considered for the role.

The London-listed company has also attracted criticism for a perceived lack of promising drugs in its pipeline, while a bribery scandal in China that led to a $489m (£343.6, €435.5m) fine in 2015 jeopardised Witty's effort to improve the company's worldwide image.

In 2015, Witty presided over the company's biggest reorganisation of the past 15 years, as he gave the green light to the sale of GSK's drug business to Novartis in return for the Swiss firm's vaccines business and cash.

Witty added it was crucial for the group to have enough time to identify a successor in order to "strongly position GSK to achieve the medium-term outlook set out to investors last year and deliver a return to core earnings growth in 2016".