GlaxoSmithKline chief executive Sir Andrew Witty will retire next year after 10 years in the role.
Sir Andrew, who started as a management trainee at the pharmaceutical giant 32 years ago, said: "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."
He explained that he wanted to give the board enough time to find a suitable successor - a search that will now be formally launched.
Valued at around £70 billion, GlaxoSmithKline is one of the most valuable companies listed in London. Last month, GSK reported a 2% rise in sales to £6.29bn for the three months to December. However, it posted a pre-tax loss of £416 million compared with a £531m profit in the same period in 2014.
At the time, Sir Andrew said the pharmaceutical firm was dedicated to restructuring its business in a "challenging" time for the healthcare market. And it was exactly that structure that came under fire from leading fund manager Neil Woodford not so long ago. In January, Mr Woodford called for GSK to be broken up, claiming its structure was like having "four FTSE 100 companies bolted together".
On Sir Andrew's retirement, GSK chairman Sir Philip Hampton, said: “Andrew’s retirement next year will represent the culmination of 32 years of service and leadership to GSK and the industry. In the meantime, we will now start a formal process to appoint his successor, whilst also ensuring the Group remains focussed on execution of its strategy to drive growth and performance.”