By Yadarisa Shabong

(Reuters) -GSK's CEO Emma Walmsley will step down at the end of the year and be replaced by insider Luke Miels, the drugmaker said on Monday, a surprise switch as the British firm looks to rev up its pipeline and navigate U.S. trade tariff uncertainty.

The move caps Walmsley's over eight years in the role that made her the most senior female CEO in the UK. She drove the spin-off of GSK's consumer healthcare business but never fully won over investors, with worries about a thinning pipeline.

The shake-up comes as GSK faces a tumultuous wider environment with punishing U.S. trade tariffs, battles over drug pricing and increased demand for new medicines to head off patent expiries and declining revenues. Companies more broadly are shaking up leadership from fashion to retail.

"Today, GSK is a biopharma innovator, with far stronger momentum and prospects than nine years ago," Walmsley, 56, said in a statement, adding that next year was key for the company and that it was the "right moment for new leadership".

A POSITIVE SHIFT FOR INVESTORS

Analysts and investors welcomed the announcement, with Miels having been widely seen as Walmsley's eventual successor. GSK's shares, which have fallen about 11% since Walmsley took over as CEO and trade at a discount to peers, rose around 2.5%.

"(The) new CEO appointment is likely to shift sentiment positively," said analysts at Bank of America Securities. "Miels has a strong pharma focus at a time where business is shifting to speciality drivers."

Walmsley will step down before her contract ends in September next year. Since taking over in 2017, she has overseen the separation of consumer healthcare group Haleon ( HLN ) and focused GSK on cancer and infectious diseases.

She also steered the company through the COVID-19 pandemic, Unilever's failed bid to buy its consumer assets, and pressures from activist shareholders Elliott and Bluebell.

Miels, who will start as CEO on January 1, will be tasked with delivering GSK's annual sales target of more than 40 billion pounds ($54 billion) by 2031, despite a challenging economic and geopolitical backdrop that includes uncertainties over U.S. trade policy.

"We see the appointment of Luke Miels ... as CEO designate as an unexpected yet welcome move," Barclays said in a note. "Major challenge will be pipeline delivery to meet the 40 billion pound topline (by 2031) target."

Miels, 50, joined GSK in 2017 and is currently chief commercial officer, overseeing  the company's global medicines and vaccines portfolio.

J.P. Morgan said in a note that the early appointment would give Miels more time to stamp his mark ahead of the patent expiry of key HIV drug dolutegravir in April 2028, allowing him to "focus on the next phase of GSK's growth".

NAVIGATING A TOUGH GLOBAL ENVIRONMENT

Miels will also need to steer GSK through a volatile global market, with U.S. trade tariffs on pharmaceuticals as President Donald Trump pushes for lower drug prices and more investment.

Earlier this month, GSK announced plans to invest $30 billion in U.S. research and development and supply chain infrastructure over five years. The investment includes $1.2 billion to build a new factory in Pennsylvania.

GSK said that Walmsley would remain at the company until the end of her contract and support the CEO transition.

"Given the potential impact to GSK's operating environment arising from geopolitics and new technologies, the Board has asked Emma to support the company and the new CEO on these matters," GSK said in the statement.

The appointment of Miels as CEO designate was the result of a "rigorous process" conducted by the board, which considered both internal and external candidates, GSK said.

Miels said in a statement that GSK had "outstanding prospects" that he would develop with "humility and ambition."

($1 = 0.7441 pounds)